TWIN (2)

twin (2). Twin, v. to bring two, pair, be paired, depart

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Twin, n. 1. 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. 2. pl. (Astron.) A sign and constellation of the zodiac; Gemini. See Gemini.
3. 3. A person or thing that closely resembles another.
4. 4. (Crystallog.) 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° 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.