The equivalence principle is the complete equality of gravitational and inertial mass, gravity, and acceleration, and the identification of free-falling frames with inertial frames. The equivalence principle is the fundamental basis for the general theory of relativity.

The weak (or Newtonian) equivalence principle is the principle that the laws of mechanics are the same in inertial frames of reference. This implies that gravitational mass and inertial mass are equivalent. The strong (or Einstein) equivalence principle is the principle that all physical laws, not just those of mechanics, are the same in all inertial and freely falling frames of reference.