1 [twin]
either of two children or animals brought forth at a birth.
either of two persons or things closely related to or closely resembling each other.
Also called twin room. a type of hotel accommodation with twin beds, for occupancy by two persons. Compare double ( def 13 ).
Also called hemitrope. Crystallography. a compound crystal consisting of two or more parts or crystals definitely oriented each to the other; macle.
Twins, Astronomy, Astrology. the constellation or sign of Gemini.
being a twin or twins: twin sisters.
being two persons or things closely related to or closely resembling each other.
being one of a pair; identical: a twin bracelet; a twin peak.
consisting of two similar parts or elements joined or connected: a twin vase.
Botany, Zoology. occurring in pairs; didymous.
Crystallography. of the nature of a twin; hemitrope.
twofold or double.
verb (used with object), twinned, twinning.
to bring together in close relationship; pair; couple.
to furnish a counterpart to or a replica of; match.
Informal. to divide or convert into two, parts, similar items, etc.: The old movie palace will be twinned, making two smaller theaters.
Informal. to link or combine with: The new grocery store is twinned with a restaurant.
Crystallography. to form into a twin.
Obsolete. to give birth to as twins.
verb (used without object), twinned, twinning.
to give birth to twins.
to be paired or coupled.

before 900; Middle English; Old English twinn (adj.), getwinn (noun and adj.); akin to Old Frisian twīne, Old Norse tvinnr double, Gothic twaihnai Unabridged


2 [twin]
verb (used with object), verb (used without object), twinned, twinning. Scot. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
twin (twɪn)
1.  a.  either of two persons or animals conceived at the same time
 b.  identical See also fraternal (as modifier): a twin brother
2.  a.  either of two persons or things that are identical or very similar; counterpart
 b.  (as modifier): twin carburettors
3.  Also called: macle a crystal consisting of two parts each of which has a definite orientation to the other
vb , twins, twinning, twinned
4.  to pair or be paired together; couple
5.  (intr) to bear twins
6.  (intr) (of a crystal) to form into a twin
7.  archaic (intr) to be born as a twin
8.  (tr)
 a.  to create a reciprocal relation between (two towns in different countries); pair (a town) with another in a different country
 b.  (intr) (of a town) to be paired with a town in a different country
[Old English twinn; related to Old High German zwiniling twin, Old Norse tvinnr double]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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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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Encyclopedia Britannica


either of two young who are simultaneously born from one mother. Twinning, common in many animals, is of two biological kinds: the one-egg (monozygotic), or identical, type and the two-egg (dizygotic), or fraternal, type. The latter type is more usual and can be thought of simply as a litter of two. In humans, psychological studies of sets of identical twins, since they are genetically identical, have provided much otherwise unobtainable information on the relative effects of genetic endowment and environment. See also multiple birth.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
While no record remains of what happened to her twin or her other brother.
In addition to routinely rejecting a twin, panda moms have been known to ignore
  single cubs.
It also terrifies my cats and my three-year-old twin boys.
Christine made her entrance after he'd tossed the last suitcase onto the brown
  metal twin.
Image for twin
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