one who, or that which, heads nails, rivets, etc., esp. a machine for heading.-one who heads a movement, a party, or a mob; head; chief; leader.-a brick or stone laid with its shorter face or head in the surface of the wall.-in framing, the piece of timber fitted between two trimmers, and supported by them, and carrying the ends of the tailpieces.-a reaper for wheat, that cuts off the heads only.-a fall or plunge headforemost, as while riding a bicycle, or in bathing; as, to take a header.