born two at one birth.
closely or intimately associated, joined, or united; coupled; paired.

1600–10; twin1 + -ed2

untwinned, adjective
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Word Origin & History

O.E. twinn "consisting of two, twofold, double," probably ultimately from P.Gmc. *twinjaz (cf. O.N. tvinnr, O.Dan. tvinling, Du. tweeling, Ger. zwillung), from PIE *dwisno- (cf. L. bini "two each," Lith. dvynu "twins"), from *dwi- "double," from base *dwo- "two" (see two). The
verb meaning "to combine two things closely" is recorded from c.1394. The noun developed from O.E. getwinn "double."
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

twin (twĭn)
One of two offspring born at the same birth. adj.

  1. Being two or one of two offspring born at the same birth.

  2. Consisting of two identical or similar parts; double.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
twin   (twĭn)  Pronunciation Key 
  1. One of two offspring born of a single gestation. Identical twins result from the division of a fertilized egg. Fraternal twins result from the fertilization of two separate eggs at the same time.

  2. A crystal structure consisting of two intergrown crystals that are mirror images of each other. Mineral twins can form as result of defective crystal growth in response to stress from rock deformation or during magma cooling.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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