A @L03493@ that carries away the bonding electron pair. For example, in the @H02902@ of an alkyl chloride, Cl is the nucleofuge. The tendency of atoms or groups to depart with the bonding electron pair is called nucleofugality. The adjective is nucleofugal.
See also:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1145 [Terms] [Paper]