The fundamental principle of the theory of relativity, proposed by Albert Einstein, is that the laws of physics are the same for all observers in uniform motion relative to one another. This principle leads to concepts such as time dilation and length contraction, which illustrate how measurements of space and time are affected by relative motion. Additionally, it predicts that the speed of light is constant in a vacuum for all observers, fundamentally altering our understanding of gravity and the fabric of space-time.