The word quaternion was coined by the Irish physicist-astronomer-mathematician William Rowan Hamilton (1788-1856), and is a generalised complex number consisting of four components.
a + bi + cj + dk
Where a,b,c and d are Real numbers and
ij = -ji = k; jk = -kj = i; ki = -ik = j
and i² = j² = k² = ijk = -1