re·entrant I. \(ˈ)rē+\adjective 1.: directed inward 2.: having its ends closed on itself < reentrant armature winding > II. noun 1.: one that reenters or is reentrant 2. a.: reentering angle < a reentrant in a fortification > b.: an indentation between two salients in a horizontal plane < a reentrant in a coastline >