Depends on whom you ask. Newton would say: relative to whom? Or if you want an absolute fixed frame of reference, then how have you determined it relative to the rest of the universe?

Mach would have said "relative to the fixed stars", which is convenient but vague; most measures suggest that our frame of reference is pretty well symmetrical with the fixed stars in all directions, which  is at least a convenient explanation for the Michelson-Morley results.

Einstein might have said: your own frame of reference is stationary and for you it is the reference by which you measure everything else. You also will find that the one thing that you and all other observers agree on, is the speed of light in empty space. 

Simple, no?