Quaternion is a 4D complex number representing rotation in 3D space. It has the form of: w + xi + yj + zk, where w is the real scalar value and x, y, z are the

Quaternion is a 4D complex number representing rotation in 3D space. It has the form of: w + xi + yj + zk, where w is the real scalar value and x, y, z are the values of the i, j, k imaginary axes. Complex number is a natural fit for rotation because operations of unit complex numbers end up on the same complex unit circle/sphere. e.g. Multiplying two quaternions performs two rotations one after another. Of course, there's the octonion complex number with 8 axes for representing rotation in 4D hyperspace.