The phenomenon describing the slowing of time for an object moving at relativistic speeds is time dilation.
In special relativity, an observer measures a moving clock as ticking more slowly than an identical clock at rest in that observer’s frame. The effect grows rapidly as speed approaches the speed of light, 299,792,458 metres per second. The Lorentz factor quantifies the difference: at everyday speeds it is imperceptibly small, but it matters for fast particles and spacecraft.
Einstein’s 1905 theory connected time dilation to the requirement that every inertial observer measure the same speed of light. A light-clock thought experiment makes the logic visible: motion lengthens the light’s path, so each tick takes longer for the outside observer.
Time dilation is often confused with gravitational time dilation, which results from differences in gravitational potential and is predicted by general relativity. Relativistic mass increase and length contraction are separate effects. Both observers in uniform relative motion can measure the other’s clock as slow; reunions, such as in the twin paradox, require careful treatment of changing frames.