The atom which replace hydrogen frm long carbon chain and confer specific properties to that hydrocarbon regardless of length of hydrocarbon
An atom or a group of atom that become the activity side of molecule

Functional group can be defined as an atom or group of atoms which makes a carbon compound reactive and decides its property.
Functional group may be define as atom or group of atom which are responsible for most of the character stick property of chemical compound
Functional group is defined as an atom or a group of atoms , present in a molecule which largely determines its chemical properties