What is time dilation?

In the theory of special relativity, the slowing down of a clock as determined by an observer who is in relative motion with respect to that clock, is called time dilation. In this phenomenon, a person finds a clock (or other time device) similar or identical to their own, ticking at a slower pace, making the observer believe that time has “slowed down” for the other clock but in reality, time passes at a constant rate. For example, as a spacecraft approaches, to an observer on earth, if he watches the clock on a spacecraft, it will seem that the clock is ticking slower than their own, as a result of relative motion.