Monovalen radikal yang terjadi bila suatu hidrokarbon alifatik hilang satu atom hidrogennya.

Merriam-Webster Online Dictionary
alkyl (adjective)
having a monovalent organic group and especially one CH (as methyl) derived from an alkane (as methane) - n 2n +1
alkyl (noun)
a compound of one or more alkyl groups with a metal - mercury alkyls
Alkyl (Wikipedia)
Isopropyl group
Methyl group

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 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−.