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.
3. A person or thing that closely resembles another.
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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