Accelerated frames

Or non-inertial frames.

From here: The issue is not so much that it is difficult in principle to construct a reference frame for a non-inertial object, but rather there is just no one way that is accepted as the standard way. So if Bob is inertial then the phrase "Bob's frame" has an accepted standard meaning. If Bob is non-inertial then the phrase "Bob's frame" can be defined any number of ways, subject only to the requirements that the coordinates must be smooth and invertible.