In the theory of special relativity , the "slowing down" of a clock as perceived by an observer in relative motion with respect to that clock.
Time dilation becomes noticeable only at speeds approaching that of light and has been accurately confirmed by the apparent increased lifetime of unstable subatomic particle s traveling at nearly the speed of light and by the precise timing of atomic clocks carried on airplanes. See also Lorentz-FitzGerald contraction .