In particular, a marsh or fen formed when the litter of decaying reeds (e.g. in a lake) raises the ground level above the water, allowing more vegetation like sedges and then low bushes or trees to grow; a marshy woodland. (Compare marsh, swamp, bog, fen.)
From Middle English kerr and Middle English carr meaning meadow, field or grassland of a low lying variety, itself from Old English carr; possibly related to Old Norse kjarr. Compare Swedish kärr, Icelandic kjarr.