Matrices and Linear Transformations - 3D Math Primer for Graphics and Game Development - page 139

Figure 5.1
Rotation about the
origin in 2D

5.1.2 3D Rotation about Cardinal Axes

In 3D, rotation occurs about an axis rather than a point, with the term axis
taking on its more commonplace meaning of a line about which something
rotates. An axis of rotation does not necessarily have to be one of the
cardinal x, y, or z axes—but those special cases are the ones we consider

Figure 5.2
Rotating about the x -axis
in 3D