In physics, the term relativity is used in several related contexts:

• Galileo first developed the principle of relativity, being the postulate that the laws of physics should take the same form for all observers in uniform motion with respect to each other. It incorporates the classical view that time is universally the same for all such observers. The same principle holds in Newtonian mechanics.
• Einstein's theory of relativity consists of special relativity and general relativity. It reshaped the classical view, arguing that time is relative to the observer, in an analogous way to position in space. There is ample experimental evidence that this is indeed the case, however paradoxical the implications may appear.

Galileo's relativity principle can be viewed as a very accurate approximation at velocities small compared to the speed of light in vacuum. This accounts for the widespread validity of so-called "non-relativistic" mechanics, in such circumstances. However, strictly speaking, Newtonian mechanics is also relativistic, in that Galilean relativity incorporates many of the essential features of relativity.

The physical reason that Einstein relativity took over from Galilean relativity is basically that it is a more general framework, which includes the previous theories of mechanics as a limiting case, for small velocities.

Historically, however, the first theory to reach the fully relativistic stage of development was electromagnetism, in the form given to it by J.C. Maxwell (the so-called Maxwell equations). It is an illuminating incident in the history of science, that this true relativity was not recognized for decades. Rather, Galilean relativity was considered canonical, because the established science of mechanics supported it, so thoroughly as it did at that time. It was therefore thought that, since electromagnetism is not Galilean relativistic, this should imply that it belongs to some sort of universal, but so far undetected physical medium (the so-called "ether"). This in order to account for the optical phenomena of light, which were recognized as due to electromagnetic waves. After all, it was a new and largely untested theory at the time.

One significant test was the Michelson-Morley experiment, which did not find any evidence of the ether. By 1905, H. Poincaré, H.A. Lorentz, and others had already investigated the coordinate transformations that relate various observers (discovering the Lorentz transformation, but misinterpreting its implications). It was Einstein who took the decisive step, to realize that electromagnetism has the correct form of relativity, not Newtonian mechanics, despite all the evidence supporting it. Consequently, it was Newtonian mechanics that needed to be modified, generalized, to become fully relativistic. While electromagnetism today still stands in the original fully relativistic form.

At any rate, the term "relativity" should not be confused with relativism. The theory of relativity is a piece of objective science with very concrete, testable consequences, while the purpose of relativism is very different, namely to question all universal truths.