(1)a toroidal shape; "a ring of ships in the harbor"; "a halo of smoke"
(2)ドーナツの形
(3)anything with a round or oval shape (formed by a curve that is closed and does not intersect itself)
(4)円形または楕円形のもの（曲線で形成され、閉じていてそれ自身と交差しない）