Pronunciation: (kwu-tûr'nē-un),[key] —n. 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.