The Collaborative International Dictionary of English v.0.48:

Twin \Twin\, v. i.
   To depart from a place or thing. [Obs.] "Ere that we farther
   twin." --Chaucer.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Twin \Twin\, v. i. [imp. & p. p. Twinned; p. pr. & vb. n.
   Twinning.]
   1. To bring forth twins. --Tusser.
      [1913 Webster]

   2. To be born at the same birth. --Shak.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Twin \Twin\, v. t.
   1. To cause to be twins, or like twins in any way. --Shak.
      [1913 Webster]

            Still we moved
            Together, twinned, as horse's ear and eye.
                                                  --Tennyson.
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   2. To separate into two parts; to part; to divide; hence, to
      remove; also, to strip; to rob. [Obs.]
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            The life out of her body for to twin. --Chaucer.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Twin \Twin\, n.
   1. One of two produced at a birth, especially by an animal
      that ordinarily brings forth but one at a birth; -- used
      chiefly in the plural, and applied to the young of beasts
      as well as to human young.
      [1913 Webster]

   2. pl. (Astron.) A sign and constellation of the zodiac;
      Gemini. See Gemini.
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   3. A person or thing that closely resembles another.
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   4. (Crystallog.) A compound crystal composed of two or more
      crystals, or parts of crystals, in reversed position with
      reference to each other.
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   Note: The relative position of the parts of a twin may be
         explained by supposing one part to be revolved 180[deg]
         about a certain axis (called the twinning axis), this
         axis being normal to a plane (called the twinning
         plane) which is usually one of the fundamental planes
         of the crystal. This revolution brings the two parts
         into parallel position, or vice versa. A contact twin
         is one in which the parts are united by a plane
         surface, called the composition face, which is usually
         the same as the twinning plane. A penetration twin is
         one in which the parts interpenetrate each other, often
         very irregularly. Twins are also called, according to
         form, cruciform, geniculated, etc.
         [1913 Webster]

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.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

twin
    adj 1: being two identical [syn: duplicate, matching,
           twin(a), twinned]
    n 1: either of two offspring born at the same time from the same
         pregnancy
    2: (astrology) a person who is born while the sun is in Gemini
       [syn: Gemini, Twin]
    3: a waterfall in the Snake River in southern Idaho [syn:
       Twin, Twin Falls]
    4: a duplicate copy [syn: counterpart, similitude, twin]
    v 1: duplicate or match; "The polished surface twinned his face
         and chest in reverse" [syn: twin, duplicate,
         parallel]
    2: bring two objects, ideas, or people together; "This fact is
       coupled to the other one"; "Matchmaker, can you match my
       daughter with a nice young man?"; "The student was paired
       with a partner for collaboration on the project" [syn:
       match, mate, couple, pair, twin]
    3: grow as twins; "twin crystals"
    4: give birth to twins