Twin (?), a. [OE. twin double, AS. getwinne two and two, pl., twins; akin to D. tweeling a twin, G. zwilling, OHG. zwiniling, Icel. tvennr, tvinnr, two and two, twin, and to AS. twi- two. See Twice, Two.] 1. Being one of two born at a birth; as, a twin brother or sister.
2. Being one of a pair much resembling one another; standing the relation of a twin to something else; — often followed by to or with. Shak.
3. (Bot.) Double; consisting of two similar and corresponding parts.
4. (Crystallog.) Composed of parts united according to some definite law of twinning. See Twin, n., 4.
Twin boat, orTwin ship(Naut.), a vessel whose deck and upper works rest on two parallel hulls. — Twin crystal. See Twin, n., 4. — Twin flower(Bot.), a delicate evergreen plant (Linnæa borealis) of northern climates, which has pretty, fragrant, pendulous flowers borne in pairs on a slender stalk. — Twin-screw steamer, a steam vessel propelled by two screws, one on either side of the plane of the keel.