A fen is a marshy area — a damp, grassy area of ground. If your entire back yard is a fen, you'll want to wear tall rubber boots when you walk your dog.
A fen is similar to a marsh, a mire, a swamp, or a bog. In all of these wetland ecosystems, the water level tends to rise and fall, leaving them constantly soggy to some degree. Fens are distinguished by the kinds of plants that grow there, including tall grasses and moss, and by the acidity of the water. The word fen comes from the Old English fenn, which means "mud, mire, dirt, or marsh."