Inertial reference frame
In physics, an inertial reference frame is a frame of reference in which the laws of physics take their simplest form, and the motion of objects can be explained without reference to fictitious forces.
For example, the relationship between the Earth and the sun can be described either in geocentric or heliocentric terms. In other words, one can describe the sun as revolving around the Earth or the Earth revolving around the sun, and both are equally mathematically valid. However, describing it in geocentric terms (with the sun revolving around the Earth) requires the introduction of fictitious forces to explain why the massive sun curves around the relatively tiny Earth. The heliocentric description, however, requires no such fictitious forces, and is therefore the inertial frame of reference. Thus, given the scientific principle of parsimony, inertial reference frames are preferred.
According to the theory of relativity, fictitious forces are indistinguishable from gravity. Inertial frames only make sense as local approximations, if masses are present.