Special relativity is the theory of the structure of spacetime. It was introduced in Albert Einstein's 1905 paper "On the Electrodynamics of Moving Bodies". It is based on two postulates: that the laws of physics are invariant (i.e., identical) in all inertial frames of reference (i.e., non-accelerating frames of reference), and that the speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.
Published by Albert Einstein in 1905, it reconciled Maxwell's equations for electricity and magnetism with the laws of mechanics.
A fundamental theory of physics, with wide-ranging consequences in many areas, including the famous equation E=mc².