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