时间膨胀是一种物理现象：两个完全相同的时钟之中，拿着甲钟的人会发现乙钟比自己的走得慢。这现象常被说为是对方的钟“慢了下来”，但这种描述只会在观测者的参考系上才是正确的。任何本地的时间（也就是位于同一个座标系上的观测者所测量出的时间）都以同一个速度前进。时间膨胀效应适用于任何解释时间速度变化的过程。 在阿尔伯特·爱因斯坦的相对论中，时间膨胀出现于两种状况：

狭义相对论中，时间膨胀效应是相互性的：从任一个时钟观测，都是对方的时钟走慢了（当然我们假定两者相互的运动的等速均匀的，两者在观测对方时都没有加速度）。

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.

An accurate clock at rest with respect to one observer may be measured to tick at a different rate when compared to a second observer's own equally accurate clock. This effect arises neither from technical aspects of the clocks nor from the fact that signals need time to propagate, but from the nature of spacetime itself.