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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 Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and ...
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
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 The International Organization for Standardization (ISO ) is an international standard An international standard is a technical standard A technical standard is an est ...
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 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 math ...
(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 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 a ...
is an identifier, but it is not a ''unique'' identifier—for that, a
serial number A serial number is a unique identifier A unique identifier (UID) is an identifier 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 ...

serial number
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
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 A morpheme is the smallest meaningful lexical item in a language. A morpheme is not a word. The difference between a morpheme and a word is that a morpheme bound and free morphemes, sometimes does not stand alone ...
ous; but they are not always connotatively synonymous, because
code name A code name, call sign or cryptonym is a Code word (figure of speech), 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 ...
s and ID numbers are often connotatively distinguished from names in the sense of traditional
natural language In neuropsychology Neuropsychology is a branch of psychology. It is concerned with how a person's cognition and behavior are related to the brain and the rest of the nervous system. Professionals in this branch of psychology often focus on ...
naming. For example, both "
Jamie Zawinski Jamie Zawinski (born November 3, 1968), commonly known as jwz, is an American impresario An impresario (from the Italian ''impresa'', "an enterprise or undertaking") is a person who organizes and often finances concerts, plays, or opera O ...
" and "
Netscape Netscape Communications Corporation (originally Mosaic Communications Corporation) was an American independent computer services company with headquarters in Mountain View, California Mountain View is a city in Santa Clara County, Californi ...

Netscape
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 sciences, social and behavioral sciences, emic () and etic () refer to two kinds of field research done and viewpoints obtained: ''emic'', from within the social group (from the perspective of the ...
indistinction rather than an
etic In anthropology Anthropology is the of ity, concerned with , , , and , in both the present and past, including . studies patterns of behaviour, while studies cultural meaning, including norms and values. studies how language influenc ...
one.


Metadata

In
metadata Metadata is "data Data (; ) are individual facts, statistics, or items of information, often numeric. In a more technical sense, data are a set of values of qualitative property, qualitative or quantity, quantitative variable (research), v ...

metadata
, an identifier is a language-independent label, sign or token that uniquely identifies an object within an identification scheme. 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 dictionary, data di ...
when naming a
data elementIn metadata Metadata is " data" that provides information about other data". In other words, it is "data about data". Many distinct types of metadata exist, including descriptive metadata, structural metadata, administrative metadata, reference m ...
. ID codes may inherently carry
metadata Metadata is "data Data (; ) are individual facts, statistics, or items of information, often numeric. In a more technical sense, data are a set of values of qualitative property, qualitative or quantity, quantitative variable (research), v ...

metadata
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 theoryThe theory of statistics provides a basis for the whole range of techniques, in both study design and data analysis Data analysis is a process of inspecting, Data cleansing, cleansing, Data transformation, transforming ...
). Opaque identifiers—identifiers designed to avoid leaking even that small amount of information—include "really
opaque pointerIn computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a specific task. Programming involves tasks such as: analysis, generatin ...
s" and Version 4 UUIDs.


In computer science

In
computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of computation, automation, a ...
, identifiers (IDs) are lexical tokens that name
entities Entity may refer to: Computing * Character entity reference, replacement text for a character in HTML or XML * Entity class, a thing of interest within an entity–relationship model or diagram * SGML entity, a primitive data type in Standard Gen ...
. Identifiers are used extensively in virtually all
information processing system An information processor or information processing system, as its name suggests, is a system A system is a group of interacting Interaction is a kind of action that occurs as two or more objects have an effect upon one another. The idea ...
s. Identifying entities makes it possible to refer to them, which is essential for any kind of symbolic processing.


In computer languages

In
computer languageComputer language is a formal language In logic, mathematics, computer science, and linguistics, a formal language consists of string (computer science), words whose symbol (formal), letters are taken from an alphabet (computer science), alphabet ...
s, identifiers are tokens (also called
symbol A symbol is a mark, sign, or word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical meaning (linguistics), m ...

symbol
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 Typing is the process of writing or inputting text by pressing keys on a typewriter, computer keyboard, cell phone, or calculator. It can be distinguished from other means of text inpu ...
,
labels A label (as distinct from signage Signage is the design or use of signs and symbols to communicate a message. A signage also means signs ''collectively'' or being considered as a group. The term ''signage'' is documented to have been popula ...
,
subroutine In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a particular task. Programming involves tasks such as analysis, gener ...
s, and
packages Package may refer to: Enclosures * Packaging and labeling Packaging is the art Art is a diverse range of (products of) human activities involving creative imagination to express technical proficiency, beauty, emotional power, or concep ...
.


Ambiguity


Identifiers (IDs) versus Unique identifiers (UIDs)

Many resources 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 OrganizationsNational Adhering Organizations in chemistry are the organizations that work as the authoritative power over chemist ...
names;
INN Inns are generally establishments or buildings where travelers can seek lodging Lodging refers to the use of a short-term dwelling In law, a dwelling (also known as a residence or an abode) is a self-contained unit of accommodation ...
generic drug names versus USAN 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 communication Communication (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', mean ...

code
s and
nomenclatural
nomenclatural
systems originate within a small
namespace In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and sof ...

namespace
. 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 A morpheme is the smallest meaningful lexical item in a language. A morpheme is not a word. The difference between a morpheme and a word is that a morpheme bound and free morphemes, sometimes does not stand alone ...
ity, translation/
transcoding Transcoding is the direct digital-to-digital conversion of one encoding In communication Communication (from Latin ''communicare'', meaning "to share") is the act of developing Semantics, meaning among Subject (philosophy), entities or Org ...
, 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 Extensibility is a software engineering and systems design principle that provides for future growth. Extensibility is a measure of the ability to extend a system A system is a group of Interaction, interacting or interrelated elements that ...

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. 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. CODEN ...
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 Neuropsychology is a branch of psychology. It is concerned with how a person's cognition and behavior are related to the brain and the rest of the nervous system. Professionals in this branch of psychology often focus on ...
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 Programming language concepts Metadata