Tiempo propio es el tiempo medido para un observador que está viajando por el espacio-tiempo a una cierta velocidad. El concepto de tiempo propio es necesario en las teorías de la relatividad de Einstein para describir efectos tales como la dilatación del tiempo. La dilatación del tiempo ha sido observada y medida en diversos experimentos, y revela que el tiempo que mide un observador en movimiento uniforme respecto a otro mide un intervalo de tiempo más pequeño que el que está en reposo (aunque la perspectiva del observador en movimiento es que es el otro quien mide un intervalo más pequeño, cosa que conduce a la paradoja de los gemelos).

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In relativity, proper time along a timelike world line is defined as the time as measured by a clock following that line. It is thus independent of coordinates, and a Lorentz scalar. The proper time interval between two events on a world line is the change in proper time. This interval is the quantity of interest, since proper time itself is fixed only up to an arbitrary additive constant, namely the setting of the clock at some event along the world line. The proper time between two events depends not only on the events but also the world line connecting them, and hence on the motion of the clock between the events. It is expressed as an integral over the world line. An accelerated clock will measure a smaller elapsed time between two events than that measured by a non-accelerated (inertial) clock between the same two events. The twin paradox is an example of this effect. In terms of four-dimensional spacetime, proper time is analogous to arc length in three-dimensional (Euclidean) space. By convention, proper time is usually represented by the Greek letter τ (tau) to distinguish it from coordinate time represented by t. By contrast, coordinate time is the time between two events as measured by an observer using that observer's own method of assigning a time to an event. In the special case of an inertial observer in special relativity, the time is measured using the observer's clock and the observer's definition of simultaneity. The concept of proper time was introduced by Hermann Minkowski in 1908, and is a feature of Minkowski diagrams.

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