Substituent

In organic chemistry and biochemistry, substituent is the name for a single atom or for a group of atoms that take the position of another atom in a molecule. They replace this atom. This reaction is called a substitution reaction. The atom or group of atoms that gets replaced is called the leaving group.
The terms substituent, side chain, group, branch, or pendant group are used in the same way to describe branches from a parent structure. These terms have different meanings in polymer chemistry. In polymers, side chains extend from a backbone structure, while they are attached to the alpha carbon atoms of the amino acid backbone in proteins. 
Organic chemistry has rules for naming compounds with substituent groups. The substituent group is listed first with a suffix to describe how it is attached to the main carbon chain. The suffix -yl is used when naming organic compounds that contain a single bond replacing one hydrogen. -ylidene and -ylidyne are used for double bonds and triple bonds respectively. When naming hydrocarbons that contain a substituent, position numbers may be used. These number will indicate which carbon atom the substituent is attached to. One case where this information is needed is that of isomers.
The phrases most-substituted and least-substituted are often used to describe molecules and predict their products. For example:
Structures.
In a structural diagram or formula, the letters R or X are used to represent a substituent group. An organic substituent such as methyl, ethyl, aryl can be written as R (or R1, R2, etc.) This is a generic placeholder. The "R" derived from "radical" or "rest". The R can represent as many atoms as the author wants. The first to use this symbol was Charles Frédéric Gerhardt in 1844.
The symbol X can represent electronegative substituents such as the halides.