Source : Webster's Revised Unabridged Dictionary (1913)

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.

Twinning \Twin"ning\, n. (Crystallog.)
   The assemblage of two or more crystals, or parts of crystals,
   in reversed position with reference to each other in
   accordance with some definite law; also, rarely, in
   artificial twinning (accomplished for example by pressure),
   the process by which this reversal is brought about.

   {Polysynthetic twinning}, repeated twinning of crystal
      lamell[ae], as that of the triclinic feldspars.

   {Repeated twinning}, twinning of more than two crystals, or
      parts of crystals.

   {Twinning axis}, {Twinning plane}. See the Note under {Twin},
      n.

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}]

twinning
     adj : producing two offspring at a time [syn: {biparous}]

twinning
     See {twin}

Source : Free On-Line Dictionary of Computing

twinning
     
         Keeping a {mirror} of a {magnetic tape}.
     
        (1997-07-18)