This article is from the Fractal FAQ, by Ermel Stepp email@example.com with numerous contributions by others.
Quaternions have 4 components (a+ib+jc+kd) compared to the two of
complex numbers. Operations such as addition and multiplication can be
performed on quaternions, but multiplication is not commutative..
Quaternions satisfy the rules i^2=j^2=k^2=-1, ij=-ji=k, jk=-kj=i, ki=-ik=j.
QJulia page (quaternions) (Henrik Engstrvm)