Time dilation

Time dilation is the phenomenon whereby an observer finds that another's clock which is physically identical to their own is ticking at a slower rate as measured by their own clock. This is often taken to mean that time has "slowed down" for the other clock, but that is only true in the context of the observer's frame of reference. Locally (i.e., from the perspective of any observer within the same frame of reference, without reference to another frame of reference), time always passes at the same rate. The time dilation phenomenon applies to any process that manifests change over time.
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Time passes more slowly within a frame of reference travelling at speed than it does in the surrounding universe. Time is slowed by about 40 nanoseconds in a flight from London to New York but the effect is more easily observed at (relativistic) speeds approaching that of light. This is predicted by special relativity. Time will also pass more slowly in a strong gravitational field than in a weak one. See also length contraction, mass dilation.