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.

3. <botany> Double; consisting of two similar and corresponding parts.

4. <chemistry> Composed of parts united according to some definite law of twinning. See Twin. Twin boat, or Twin ship, a vessel whose deck and upper works rest on two parallel hulls. Twin crystal. See Twin.

<botany> Twin flower, a delicate evergreen plant (Linnaea 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.

Origin: 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. 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. <astronomy> A sign and constellation of the zodiac; Gemini. See Gemini.

3. A person or thing that closely resembles another.

4. <chemistry> A compound crystal composed of two or more crystals, or parts of crystals, in reversed position with reference to each other.

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.

(01 Mar 1998)

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