functional group

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An atom or group of atoms, such as a carboxyl group, in an organic compound that defines the structure of a family of compounds and determines the properties of the family.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun organic chemistry A specific grouping of elements that is characteristic of a class of compounds, and determines some properties and reactions of that class.
• noun ecology A collection of organisms of specific morphological, physiological, behavioral, biochemical properties.