In organic chemistry, functional groups are lexicon specific groups of atoms or bonds within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction(s) regardless of the size of the molecule it is a part of. However, its relative reactivity can be modified by nearby functional groups.

The word moiety ( /ˈmɔɪəti/) is often used synonymously to "functional group," but, according to the IUPAC definition, a moiety is a part of a molecule that may include either whole functional groups or parts of functional groups as substructures. For example, an ester (RCOOR') has an ester functional group (COOR) and is composed of an alkoxy moiety (-OR') and an acyl moiety (RCO-), or, equivalently, it may be divided into carboxylate (RCOO-) and alkyl (-R') moieties. Each moiety may contain additional functional groups--for example, methyl para-hydroxybenzoate contains a phenol functional group within the acyl moiety.