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.

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

   2. To be born at the same birth. --Shak.

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

            Still we moved Together, twinned, as horse's ear and
            eye.                                  --Tennyson.

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

            The life out of her body for to twin. --Chaucer.

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.

   2. pl. (Astron.) A sign and constellation of the zodiac;
      Gemini. See {Gemini}.

   3. A person or thing that closely resembles another.

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

   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.

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

Source : WordNet®

twin
     adj 1: being two identical [syn: {duplicate}, {matching}, {twin(a)},
             {twinned}]
     2: very similar [syn: {siamese}]
     [also: {twinning}, {twinned}]

twin
     v 1: duplicate or match; "The polished surface twinned his face
          and chest in reverse" [syn: {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}]
     3: grow as twins; "twin crystals"
     4: give birth to twins
     [also: {twinning}, {twinned}]

twin
     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 in in Gemini
        [syn: {Gemini}]
     3: a waterfall in the Snake River in southern Idaho [syn: {Twin
        Falls}]
     4: a duplicate copy [syn: {counterpart}, {similitude}]
     [also: {twinning}, {twinned}]