quaternion (kwətûrˈnēən) [key], in mathematics, a type of higher complex number first suggested by Sir William R. Hamilton in 1843. A complex number is a number of the form a + bi when a and b are real numbers and i is the so-called imaginary unit defined by the equation i 2 = - 1. The rules for operating with complex numbers are simply those of operating with the polynomial a + bx except that i 2 is replaced by - 1 whenever it occurs. A quaternion, an extension of this concept, is a number of the form a + bi + cj + dk when a, b, c, and d are real numbers and i, j, and k are imaginary units defined by the equations i 2 = j 2 = k 2 = ijk = - 1. Quaternions, as well as vectors and tensors (later outgrowths of the concept of quaternions), have many important applications in mechanics.
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