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.
      [1913 Webster]

   2. To separate into two parts; to part; to divide; hence, to
      remove; also, to strip; to rob. [Obs.]
      [1913 Webster]

            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.
      [1913 Webster]

   3. A person or thing that closely resembles another.
      [1913 Webster]

   4. (Crystallog.) A compound crystal composed of two or more
      crystals, or parts of crystals, in reversed position with
      reference to each other.
      [1913 Webster]

   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.
      [1913 Webster]

   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

Moby Thesaurus II by Grady Ward, 1.0:

190 Moby Thesaurus words for "twin":
   Doppelganger, Janus-like, accessory, accompanying, actual thing,
   alike, all one, all the same, ally, alter ego, ambidextrous,
   analogon, analogous, analogue, associate, associated, attendant,
   attending, bifacial, bifold, biform, bifurcated, bilateral, binary,
   binate, biparous, bipartisan, bipartite, bivalent, brother,
   carbon copy, clone, close copy, close match, coequal, cognate,
   coincident, collateral, combine, combined, companion, comparable,
   compeer, complement, concomitant, concurrent, conduplicate,
   congenator, congener, conjoint, connect, consubstantial,
   coordinate, copy, correlate, correlative, correspondent,
   corresponding, counterpart, couple, coupled, dead ringer,
   dichotomous, disomatous, ditto, double, double-faced, duadic, dual,
   dualistic, dupe, duple, duplex, duplicate, duplicated, dyadic,
   effigy, equal, equipollent, equivalent, exact counterpart,
   exact likeness, exactly alike, facsimile, fellow, geminate,
   geminated, homograph, homonym, homoousian, homophone, icon, idem,
   identic, identical, identical same, idol, image, indistinguishable,
   ingeminate, join, joined, joint, just alike, kindred spirit, like,
   likeness, link, living image, living picture, match, matched,
   matching, mate, miniature, mirroring, model, multiply by two,
   mutual, near duplicate, no other, none other, obverse, of a kind,
   of a piece, of a size, one, opposite number, pair, paired,
   parallel, paralleling, peer, pendant, photograph, picture,
   portrait, reciprocal, redouble, reduplicate, reflection, repeat,
   replica, replicate, reproduce, resemblance, ringer, rival, rubbing,
   same, second, second self, secondary, selfsame, semblance, shadow,
   similar, similitude, simulacrum, simultaneous, sister, soul mate,
   spit and image, spitting image, such, suchlike, synonym, tally,
   the like of, the likes of, the same, the same difference, trace,
   tracing, twain, twinned, two, two-faced, two-level, two-ply,
   two-sided, two-story, twofold, undifferent, very image,
   very picture, very same, without difference, without distinction,
   yoke