The Collaborative International Dictionary of English v.0.48:

Fen \Fen\ (f[e^]n), n. [AS. fen, fenn, marsh, mud, dirt; akin to
   D. veen, OFries. fenne, fene, OHG. fenna, G. fenn, Icel. fen,
   Goth. fani mud.]
   Low land overflowed, or covered wholly or partially with
   water, but producing sedge, coarse grasses, or other aquatic
   plants; boggy land; moor; marsh.
   [1913 Webster]

         'Mid reedy fens wide spread.             --Wordsworth.
   [1913 Webster]

   Note: Fen is used adjectively with the sense of belonging to,
         or of the nature of, a fen or fens.
         [1913 Webster]

   Fen boat, a boat of light draught used in marshes.

   Fen duck (Zool.), a wild duck inhabiting fens; the
      shoveler. [Prov. Eng.]

   Fen fowl (Zool.), any water fowl that frequent fens.

   Fen goose (Zool.), the graylag goose of Europe. [Prov.
      Eng.]

   Fen land, swamp land.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fen
    n 1: 100 fen equal 1 yuan in China
    2: low-lying wet land with grassy vegetation; usually is a
       transition zone between land and water; "thousands of acres
       of marshland"; "the fens of eastern England" [syn: marsh,
       marshland, fen, fenland]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

FEN
       Free-net Erlangen/Nuernberg