Dictionary

Basic law of classical mechanics and special relativity: bodies on which no external forces act move with constant speed on straight paths. In the geometrical language of special relativity, this can be reformulated as: bodies on which no external forces act move on straight line in spacetime.

Strictly speaking, though, this law is only true in specific reference frames. This gives rise to a further, more precise reformulation: It is always possible to find a reference frame on which bodies on which no external forces act move with constant speed along straight paths. Such reference frames are called inertial frames.

In general relativity, the law of inertia holds in a somewhat modified form: there, bodies on which no external, non-gravitational forces act don’t move on straight lines through spacetime, but on geodesics.