Gimbal lock
The loss of one rotational degree of freedom when two of three rotation axes line up, so the object can no longer be turned freely in every direction.
Describe an orientation with three Euler angles and the rotations happen one after another about three axes. If the middle rotation lines the outer two up parallel, the system collapses from three degrees of freedom to two — the two remaining turns now do the same thing, and the lost direction can't be reached without moving away and back.
The term comes from mechanical gyroscopes. On Apollo 11 the guidance computer warned about gimbal lock precisely because the stabilised platform had only three rings; Michael Collins joked about getting a fourth. In graphics and robotics the problem is avoided by storing rotations as quaternions and interpolating them with slerp.