What is the name of the principle that states the laws of physics are the same in all inertial frames?

The principle stating that the laws of physics are the same in all inertial frames is the principle of relativity.

An inertial frame is a reference frame moving at constant velocity, without acceleration. The principle says that no such frame is physically privileged: experiments performed in a smoothly moving laboratory should follow the same laws as experiments performed in another inertial laboratory. Uniform motion itself cannot reveal an absolute state of rest.

Galileo expressed an early version of this idea for mechanics, and Albert Einstein extended it in 1905 as a foundation of special relativity. Einstein paired it with the postulate that light travels at the same speed in vacuum for all inertial observers. Together, these principles lead to time dilation, length contraction, and the relativity of simultaneity.

The principle of relativity is broader than the equivalence principle, which connects gravity with acceleration, and it is distinct from the uncertainty principle of quantum mechanics. It also does not say that every observer measures identical times or distances—only that the underlying physical laws have the same form.