Pronunciation: (kwu-tûr'nē-un), [key]
1. a group or set of four persons or things.
2. Bookbinding.four gathered sheets folded in two for binding together.
3. Math.
a. an expression of the form a + bi + cj + dk, where a, b, c, and d are real numbers; i2 = j2 = k2 = -1; and ij = -ji = k, jk = -kj = i, and ki = -ik = j.
b. a quantity or operator expressed as the sum of a real number and three complex numbers, equivalent to the quotient of two vectors. The field of quaternions is not commutative under multiplication.

Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.

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