In organic chemistry, an alkyl group 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 . A cycloalkyl group is derived from a cycloalkane by removal of a hydrogen atom from a ring and has the general formula .
Typically an alkyl is a part of a larger molecule. In structural formulae, the symbol R is used to designate a generic (unspecified) alkyl group. The smallest alkyl group is methyl, with the formula .