Noah Webster's Dictionary (n.) Low land overflowed, or covered wholly or partially with water, but producing sedge, coarse grasses, or other aquatic plants; boggy land; moor; marsh.

Multi-Version Concordance Fen (1 Occurrence) Job 40:21 He lieth under the shady trees, in the covert of the reed, and fens. (Root in KJV JPS ASV DBY WBS)