Source : Webster's Revised Unabridged Dictionary (1913)

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.