In organic chemistry, an alkyl substituent is an alkane missing one hydrogen. The term alkyl is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of CnH2n+1. A cycloalkyl is derived from a cycloalkane by removal of a hydrogen atom from a ring and has the general formula CnH2n-1. Typically an alkyl is a part of a larger molecule. In structural formula, the symbol R is used to designate a generic (unspecified) alkyl group. The smallest alkyl group is methyl, with the formula CH3−. 
This compound is known as 2,3,3-trimethylpentane. H
If there is more than one of the same alkyl group attached to a chain, then the prefixes are used on the alkyl groups to indicate multiples (i.e., di, tri, tetra, etc.)
This compound is known as 2,3,3-trimethylpentane. Here three identical alkyl groups attached to carbon atoms 2, 3, and 3. These numbers are included in the name to avoid ambiguity on the position of the groups, and "tri" indicates that there are three identical methyl groups. If one of the methyl groups attached to the third carbon atom were an ethyl group, then the name would become 3-ethyl-2,3-dimethylpentane. Note that when there are different alkyl groups, they are listed in alphabetical order.
In addition, each position on an alkyl chain can be described according to how many other carbon atoms are attached to it. The terms primary, secondary, primary, secondary, tertiary, and quaternary refer to a carbon attached to one, two, three, or four other carbons respectively.
The first named alkyl radical was ethyl, named so by Liebig in 1833 from the German word "aether" (which in turn had been derived from the Greek word "aither") (https://en.wiktionary.org/wiki/ether , i.e., the substance now known as diethyl ether) and the Greek word ύλη (hyle), meaning "matter". This was followed by methyl (Dumas and Peligot in 1834, meaning "spirit of wood") and amyl (Auguste Cahours in 1840). The word alkyl was introduced by Johannes Wislicenus in or before 1882, based on the German word "Alkoholradicale" and then-common suffix -yl.