What is "twin-screw steamer"

Twin \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, or Twin 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 ( Linn[ae]a 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.

A twin-screw steamer (or steamship) (TSS) is a steam-powered vessel propelled by two screw propellers, one on either side of the plane of the keel.