from The American Heritage® Dictionary of the English Language, 5th Edition.

  • noun One of two offspring born at the same birth.
  • noun One of two identical or similar people, animals, or things; a counterpart.
  • noun Mineralogy Two interwoven crystals that are mirror images of each other.
  • noun A twin-size bed.
  • adjective Being two or one of two offspring born at the same birth.
  • adjective Being two or one of two identical or similar people, animals, or things.
  • adjective Botany Of or relating to structures, such as flowers, that occur in pairs.
  • adjective Consisting of two identical or similar parts.
  • intransitive verb To give birth to twins.
  • intransitive verb Archaic To be one of twin offspring.
  • intransitive verb To be paired or coupled.
  • intransitive verb To pair or couple.
  • intransitive verb To provide a match or counterpart to.

from The Century Dictionary.

  • To couple; pair; mate; join intimately or link together; said of two united or of one joined to another.
  • Specifically, in mineralogy, to form or unite into a compound or twin crystal by a reversal of the molecular structure according to some definite law.
  • To be coupled or paired; be mated, as one with another; specifically, to be twin-born.
  • To bring forth two at a birth.
  • To be parted in twain; be divided or sundered; come apart.
  • To part; depart; go away.
  • To be separated (from) or deprived (of): as, to twin with one's gear.
  • To part in twain; sever; sunder.
  • To part, as from another person or thing; separate; sunder; especially, to deprive.
  • Two; twain.
  • Consisting of two separate, closely related, and equal members; twofold; double; specifically, consisting of or forming twins or a pair: as, twin children.
  • One, each, or either of two; one of a pair, specifically of two born at a birth: as, a twin brother or sister.
  • In botany, growing in pairs; didymous.
  • Consisting of two chief parts alike, or nearly alike, and held firmly together: as, a twin bottle; a twin vase. The plural is used in the same sense: as, twin vases.
  • In entomology, geminate: applied to spots, punctures, spines, etc., which are close together in pairs, and distant from others.
  • noun Two; twain; a pair; a couple.
  • noun One of two; one of a pair or couple linked together by a particular tie or relation; the mate, counterpart, or fellow of another; specifically, one of two creatures produced at a birth: said of the young both of hitman beings and of beasts.
  • noun A compound crystal one part of which is in a reversed position with reference to the other, as if it had been revolved through 180° about an axis (twinning-axis) perpendicular to a plane which is called the twinning-plane, and is usually a fundamental plane of the given Crystal.
  • To cut or form in pairs by cutters or chisels.

from the GNU version of the Collaborative International Dictionary of English.

  • intransitive verb To bring forth twins.
  • intransitive verb To be born at the same birth.
  • transitive verb To cause to be twins, or like twins in any way.
  • transitive verb obsolete To separate into two parts; to part; to divide; hence, to remove; also, to strip; to rob.
  • adjective Being one of two born at a birth.
  • adjective Being one of a pair much resembling one another; standing in the relation of a twin to something else; -- often followed by to or with.
  • adjective (Bot.) Double; consisting of two similar and corresponding parts.
  • adjective (Crystallog.) Composed of parts united according to some definite law of twinning. See Twin, n., 4.
  • adjective (Naut.) a vessel whose deck and upper works rest on two parallel hulls.
  • adjective See Twin, n., 4.
  • adjective (Bot.) a delicate evergreen plant (Linnæa borealis) of northern climates, which has pretty, fragrant, pendulous flowers borne in pairs on a slender stalk.
  • adjective a steam vessel propelled by two screws, one on either side of the plane of the keel.
  • noun 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.
  • noun (Astron.) A sign and constellation of the zodiac; Gemini. See Gemini.
  • noun A person or thing that closely resembles another.


from The American Heritage® Dictionary of the English Language, 4th Edition

[Middle English, from Old English twinn, twofold; see dwo- in Indo-European roots.]

from Wiktionary, Creative Commons Attribution/Share-Alike License

Old English twinn, earlier ġetwinn, from Germanic.


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