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.
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         'Mid reedy fens wide spread.             --Wordsworth.
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   Note: Fen is used adjectively with the sense of belonging to,
         or of the nature of, a fen or fens.
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   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.
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