[qua.ter.ni.on] n [ME quaternyoun, fr. LL quaternion-, quaternio, fr. L quaterni four each, fr. quater four times; akin to L quattuor four--more at four] (14c) 1: a set of four parts, things, or persons
2. a: a generalized complex number that is composed of a real number and a vector and that depends on one real and three imaginary units b pl: the calculus of quaternions