The Collaborative International Dictionary of English v.0.48:

Twin \Twin\ (tw[i^]n), 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.
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   2. Being one of a pair much resembling one another; standing
      in the relation of a twin to something else; -- often
      followed by to or with. --Shak.
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   3. (Bot.) Double; consisting of two similar and corresponding
      parts.
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   4. (Crystallog.) Composed of parts united according to some
      definite law of twinning. See Twin, n., 4.
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   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 (Linnaea
      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.
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