pool [water] . (a) A small, natural body of standing water, usually fresh; e.g. a stagnant body of water in a marsh, or a transient puddle in a depression following a rain, or a still body of water within a cave. (b) A small, quiet, rather deep reach of a stream, as between rapids or where there is little current. See also: plunge pool. (c) A body of impounded water, artificially confined above a dam or the closed gates of a lock.