The relativity of time, according to Albert Einstein's theory of relativity, refers to the idea that time is not a universal constant but can vary depending on the speed at which an object is moving and the strength of the gravitational field it is in. According to this theory, the faster an object moves or the stronger the gravitational field, the slower time passes for that object compared to an observer at rest or in a weaker gravitational field. This phenomenon has been experimentally confirmed and has important implications in physics and astronomy.