coordinate system
frame of reference
reference system
reference frame

This is true for those watching from the planetary reference frame.

Each had its own reference frame, around which all other masses seemed to move.

The surface of the Earth is a rotating reference frame.

In the reference frame of the car, the ground is moving backwards at some speed.

The speed of light is the same in all reference frames.

Real time, though, was only one possible reference frame; what about all the others?

The amount of current is particular to a reference frame.

In the special case, the sources that create the field are at rest with respect to one of the reference frames.

In some other reference frame the field will appear differently.

No charges are now at rest in the reference frame of the test charge.