What is "time dilation"

alt. (context physics English) The slowing of the passage of time experienced by objects in motion relative to an observer; measurable only at relativistic speeds. n. (context physics English) The slowing of the passage of time experienced by objects in motion relative to an observer; measurable only at relativistic speeds.

In the theory of relativity, time dilation is a difference of elapsed time between two events as measured by observers either moving relative to each other or differently situated from a gravitational mass or masses.

A clock at rest with respect to one observer may be measured to tick at a different rate when compared to a second observer's clock. This effect arises neither from technical aspects of the clocks nor from the propagation time of signals, but from the nature of spacetime.