Functional Group
Definition
A specific cluster of atoms that gives a molecule its characteristic behavior: the -OH that makes alcohols act like alcohols.
Technical definition
A functional group is an atom or group of atoms within a molecule that confers characteristic chemical reactivity, for example the hydroxyl (-OH), carboxyl (-COOH), or amino (-NH2) group. Organic chemistry is largely the study of how functional groups transform.
Example
Ethanol's -OH group makes it mix with water and undergo very different reactions than ethane, which lacks it.
Common confusion
The hydrocarbon skeleton is not the functional group. In ethanol the -OH is the functional group; the ethyl part just comes along for the ride.