Illustrated Glossary of Organic Chemistry

Leaving group (LG; nucleofuge): An atom or group of atoms which breaks away from the rest of the molecule, taking with it the electron pair which used to be the bond between the leaving group and the rest of the molecule. The bond scission occurs via heterolytic bond cleavage. A leaving group which departs from a proton is usually referred to as the conjugate base of the corresponding acid. Even though 'group' refers to two or more atoms bonded together, a leaving group composed of a single atom (such as Br-) it is still called a leaving group.

Bromide ion is the leaving group in this SN2 reaction.

Water is the leaving group in this SN1 reaction.