Answer: quaternion

AnswerCrossword Clue
QUATERNIONSet of four.
quaternionA complex number of the form w + xi + yj + zk, where w, x, y, z are real numbers and i, j, k are imaginary units that satisfy certain conditions
quaternionA set of four people or things
quaterniona set or group of four; the operation of changing one vector into another, or the quotient of two vectors