![4373 Russian license plate Toyota car](https://upload.wikimedia.org/wikipedia/commons/f/fb/4373_Russian_license_plate_Toyota_car.jpg)
An identifier is a name that identifies (that is, labels the identity of) either a unique object or a unique ''class'' of objects, where the "object" or class may be an idea, physical
countable
In mathematics, a set is countable if either it is finite or it can be made in one to one correspondence with the set of natural numbers. Equivalently, a set is ''countable'' if there exists an injective function from it into the natural numbers; ...
object (or class thereof), or physical
noncountable substance (or class thereof). The abbreviation ID often refers to identity, identification (the process of identifying), or an identifier (that is, an instance of identification). An identifier may be a word, number, letter, symbol, or any combination of those.
The words, numbers, letters, or symbols may follow an
encoding system (wherein letters, digits, words, or symbols ''stand for''
epresentideas or longer names) or they may simply be arbitrary. When an identifier follows an encoding system, it is often referred to as a code or ID code. For instance the
ISO/IEC 11179
The ISO/IEC 11179 Metadata Registry (MDR) standard is an international ISO/IEC standard for representing metadata for an organization in a metadata registry. It documents the standardization and registration of metadata to make data understandabl ...
metadata registry standard defines a code as ''system of valid symbols that substitute for longer values'' in contrast to identifiers without symbolic meaning. Identifiers that do not follow any encoding scheme are often said to be arbitrary IDs; they are arbitrarily assigned and have no greater meaning. (Sometimes identifiers are called "codes" even when they are actually arbitrary, whether because the speaker believes that they have deeper meaning or simply because they are speaking casually and imprecisely.)
The
unique identifier
A unique identifier (UID) is an identifier that is guaranteed to be unique among all identifiers used for those objects and for a specific purpose. The concept was formalized early in the development of computer science and information systems. ...
(UID) is an identifier that refers to ''only one instance''—only one particular object in the universe. A
part number
A part number (often abbreviated PN, P/N, part no., or part #) is an identifier of a particular part design or material used in a particular industry. Its purpose is to simplify reference to that item. A part number unambiguously identifies a part ...
is an identifier, but it is not a ''unique'' identifier—for that, a
serial number
A serial number is a unique identifier assigned incrementally or sequentially to an item, to ''uniquely'' identify it.
Serial numbers need not be strictly numerical. They may contain letters and other typographical symbols, or may consist enti ...
is needed, to identify ''each instance'' of the part design. Thus the ''identifier'' "Model T" identifies the ''class'' ''(model)'' of automobiles that Ford's
Model T comprises; whereas the ''unique identifier'' "Model T Serial Number 159,862" identifies one specific member of that class—that is, one particular Model T car, owned by one specific person.
The concepts of ''name'' and ''identifier'' are
denotatively equal, and the terms are thus denotatively
synonym
A synonym is a word, morpheme, or phrase that means exactly or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are all ...
ous; but they are not always
connotatively synonymous, because
code name
A code name, call sign or cryptonym is a code word or name used, sometimes clandestinely, to refer to another name, word, project, or person. Code names are often used for military purposes, or in espionage. They may also be used in industrial c ...
s and ID numbers are often connotatively distinguished from names in the sense of traditional
natural language
In neuropsychology, linguistics, and philosophy of language, a natural language or ordinary language is any language that has evolved naturally in humans through use and repetition without conscious planning or premeditation. Natural languages ...
naming. For example, both "
Jamie Zawinski
Jamie Zawinski (born November 3, 1968), commonly known as jwz, is an American computer programmer, blogger and impresario. He is best known for his role in the creation of Netscape Navigator, Netscape Mail, Lucid Emacs, Mozilla.org, and XScree ...
" and "
Netscape
Netscape Communications Corporation (originally Mosaic Communications Corporation) was an American independent computer services company with headquarters in Mountain View, California and then Dulles, Virginia. Its Netscape web browser was onc ...
employee number 20" are identifiers for the same specific human being; but normal English-language connotation may consider "Jamie Zawinski" a "name" and not an "identifier", whereas it considers "Netscape employee number 20" an "identifier" but not a "name." This is an
emic
In anthropology, folkloristics, and the social and behavioral sciences, emic () and etic () refer to two kinds of field research done and viewpoints obtained.
The "emic" approach is an insider's perspective, which looks at the beliefs, values, ...
indistinction rather than an
etic one.
Metadata
In
metadata
Metadata is "data that provides information about other data", but not the content of the data, such as the text of a message or the image itself. There are many distinct types of metadata, including:
* Descriptive metadata – the descriptive ...
, an identifier is a language-independent label, sign or token that uniquely identifies an object within an
identification scheme
In metadata, an identification scheme is used to identify unique records in a set.
If a data element is used to identify a record within a data set, the data element uses the Identifier representation term.
An identification scheme should be c ...
. The suffix "identifier" is also used as a
representation term
A representation term is a word, or a combination of words, that semantically represent the data type (value domain) of a data element. A representation term is commonly referred to as a ''class word'' by those familiar with data dictionaries. ISO ...
when naming a
data element
In metadata, the term data element is an atomic unit of data that has precise meaning or precise semantics. A data element has:
# An identification such as a data element name
# A clear data element definition
# One or more representation terms
...
.
ID codes may inherently carry
metadata
Metadata is "data that provides information about other data", but not the content of the data, such as the text of a message or the image itself. There are many distinct types of metadata, including:
* Descriptive metadata – the descriptive ...
along with them. For example, when you know that the food package in front of you has the identifier "2011-09-25T15:42Z-MFR5-P02-243-45", you not only have that data, you also have the metadata that tells you that it was packaged on September 25, 2011, at 3:42pm UTC, manufactured by Licensed Vendor Number 5, at the Peoria, IL, USA plant, in Building 2, and was the 243rd package off the line in that shift, and was inspected by Inspector Number 45.
Arbitrary identifiers might lack metadata. For example, if a food package just says 100054678214, its ID may not tell anything except identity—no date, manufacturer name, production sequence rank, or inspector number. In some cases, arbitrary identifiers such as sequential serial numbers leak information (i.e. the
German tank problem
In the statistical theory of estimation, the German tank problem consists of estimating the maximum of a discrete uniform distribution from sampling without replacement. In simple terms, suppose there exists an unknown number of items which are s ...
). Opaque identifiers—identifiers designed to avoid leaking even that small amount of information—include "really
opaque pointer In computer programming, an opaque pointer is a special case of an opaque data type, a data type declared to be a pointer to a record or data structure of some unspecified type.
Opaque pointers are present in several programming languages includi ...
s" and
Version 4 UUIDs.
In computer science
In
computer science
Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to Applied science, practical discipli ...
, identifiers (IDs) are
lexical
Lexical may refer to:
Linguistics
* Lexical corpus or lexis, a complete set of all words in a language
* Lexical item, a basic unit of lexicographical classification
* Lexicon, the vocabulary of a person, language, or branch of knowledge
* Lex ...
tokens that name
entities
An entity is something that exists as itself, as a subject or as an object, actually or potentially, concretely or abstractly, physically or not. It need not be of material existence. In particular, abstractions and legal fictions are usually ...
. Identifiers are used extensively in virtually all
information processing system
An information processor or information processing system, as its name suggests, is a private content (be it electrical, mechanical or biological) which takes information (a sequence of enumerated symbols or states) in one form and processes ( ...
s. Identifying entities makes it possible to refer to them, which is essential for any kind of symbolic processing.
In computer languages
In
computer language
A computer language is a formal language used to communicate with a computer. Types of computer languages include:
* Construction language – all forms of communication by which a human can specify an executable problem solution to a compu ...
s, identifiers are
tokens (also called
symbol
A symbol is a mark, sign, or word that indicates, signifies, or is understood as representing an idea, object, or relationship. Symbols allow people to go beyond what is known or seen by creating linkages between otherwise very different conc ...
s) which name language entities. Some of the kinds of entities an identifier might denote include
variables,
types
Type may refer to:
Science and technology Computing
* Typing, producing text via a keyboard, typewriter, etc.
* Data type, collection of values used for computations.
* File type
* TYPE (DOS command), a command to display contents of a file.
* Typ ...
,
labels
A label (as distinct from signage) is a piece of paper, plastic film, cloth, metal, or other material affixed to a container or product, on which is written or printed information or symbols about the product or item. Information printed dir ...
,
subroutine
In computer programming, a function or subroutine is a sequence of program instructions that performs a specific task, packaged as a unit. This unit can then be used in programs wherever that particular task should be performed.
Functions may ...
s, and
packages
Package may refer to:
Containers or Enclosures
* Packaging and labeling, enclosing or protecting products
* Mail, items larger than a letter
* Chip package or chip carrier
* Electronic packaging, in electrical engineering
* Automotive package, ...
.
Ambiguity
Identifiers (IDs) versus Unique identifiers (UIDs)
A resource may carry multiple identifiers. Typical examples are:
* One person with multiple names, nicknames, and forms of address (titles, salutations)
** ''For example:'' One specific person may be identified by all of the following identifiers: Jane Smith; Jane Elizabeth Meredith Smith; Jane E. M. Smith; Jane E. Smith; Janie Smith; Janie; Little Janie (as opposed to her mother or sister or cousin, Big Janie); Aunt Jane; Auntie Janie; Mom; Grandmom; Nana; Kelly's mother; Billy's grandmother; Ms. Smith; Dr. Smith; Jane E. Smith, PhD; and Fuzzy (her jocular nickname at work).
* One document with multiple versions
* One substance with multiple names (for example,
CAS index names versus
IUPAC
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
names;
INN
Inns are generally establishments or buildings where travelers can seek lodging, and usually, food and drink. Inns are typically located in the country or along a highway; before the advent of motorized transportation they also provided accommo ...
generic drug names versus
USAN
280px, "Samguk Sagi" Book 04. Silla's Records. In 512, Usan-guk(于山國)was Ulleungdo(鬱陵島)
Usan-guk, or the State of Usan, occupied Ulleung-do and the adjacent islands during the Korean Three Kingdoms period. According to th ...
generic drug names versus brand names)
The inverse is also possible, where multiple resources are represented with the same identifier (discussed below).
Implicit context and namespace conflicts
Many
code
In communications and information processing, code is a system of rules to convert information—such as a letter, word, sound, image, or gesture—into another form, sometimes shortened or secret, for communication through a communication ...
s and
nomenclatural
Nomenclature (, ) is a system of names or terms, or the rules for forming these terms in a particular field of arts or sciences. The principles of naming vary from the relatively informal conventions of everyday speech to the internationally ag ...
systems originate within a small
namespace
In computing, a namespace is a set of signs (''names'') that are used to identify and refer to objects of various kinds. A namespace ensures that all of a given set of objects have unique names so that they can be easily identified.
Namespaces ...
. Over the years, some of them bleed into larger namespaces (as people interact in ways they formerly hadn't, e.g., cross-border trade, scientific collaboration, military alliance, and general cultural interconnection or assimilation). When such dissemination happens, the limitations of the original naming convention, which had formerly been latent and moot, become painfully apparent, often necessitating
retronym
A retronym is a newer name for an existing thing that helps differentiate the original form/version from a more recent one. It is thus a word or phrase created to avoid confusion between older and newer types, whereas previously (before there were ...
y,
synonym
A synonym is a word, morpheme, or phrase that means exactly or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are all ...
ity,
translation/
transcoding
Transcoding is the direct digital-to-digital conversion of one encoding to another, such as for video data files, audio files (e.g., MP3, WAV), or character encoding (e.g., UTF-8, ISO/IEC 8859). This is usually done in cases where a target devic ...
, and so on. Such limitations generally accompany the shift away from the original context to the broader one. Typically the system shows implicit context (context was formerly assumed, and narrow), lack of capacity (e.g., low number of possible IDs, reflecting the outmoded narrow context), lack of
extensibility (no features defined and reserved against future needs), and lack of specificity and disambiguating capability (related to the context shift, where longstanding uniqueness encounters novel nonuniqueness). Within computer science, this problem is called
naming collision
A naming collision is a circumstance where two or more identifiers in a given namespace or a given scope cannot be unambiguously resolved, and such unambiguous resolution is a requirement of the underlying system.
Example: XML element names
I ...
. The story of the origination and expansion of the
CODEN
CODEN – according to ASTM standard E250 – is a six-character, alphanumeric bibliographic code that provides concise, unique and unambiguous identification of the titles of periodicals and non-serial publications from all subject areas.
CODE ...
system provides a good case example in a recent-decades, technical-nomenclature context. The capitalization variations seen with
specific designators reveals an instance of this problem occurring in
natural language
In neuropsychology, linguistics, and philosophy of language, a natural language or ordinary language is any language that has evolved naturally in humans through use and repetition without conscious planning or premeditation. Natural languages ...
s, where the proper noun/common noun distinction (and its complications) must be dealt with. A universe in which every object had a UID would not need any namespaces, which is to say that it would constitute one gigantic namespace; but human minds could never keep track of, or semantically interrelate, so many UIDs.
Identifiers in various disciplines
See also
References
{{Authority control
Metadata