A fen is a type of wetland fed by groundwater and runoff, and containing peat below the waterline. It is, in many ways, intermediate between a bog and a marsh. Fen is an Old English word considered to have proto-Germanic origins, since it has cognates in Gothic, Old Frisian, Dutch and German.

See also the English district of East Anglia called The Fens.

Fen is an invented alternate plural of fan, and is used by science fiction, fantasy, and anime enthusiasts to distinguish themselves from "fans" such as football fans, originating by analogy to "men" being the plural of "man".

FEN is an acronym for Forsyth-Edwards Notation for chess positions.

FEN is an acronym for the Far East Network, a former name for those AFN radio and television stations in Japan, Korea and other parts of East Asia. Also, it was the name of a magazine for Japanese fans of the Far East Network.

Fen is a unit of Chinese money.

The renminbi is the legal tender in the mainland of the People's Republic of China. The base unit of the renminbi is the yuan. One yuan is divided into 10 jiao (角). One jiao is divided into 10 fen (分). The largest denomination of the renminbi is the 100 yuan note. The smallest is the 1 fen coin or note.