Spacetime
The space we live in comprises three dimensions: every movement can be decomposed into three distinct movements upwards/downwards, left/right and forwards/backwards. Since Einstein's theory of relativity, time has been added to form the four-dimensional spacetime. The time coordinate differs fundamentally from the other three: while backward movement is possible in space, time only goes forward. However, time is relative, just as space: time elapses differently from one observer to the other.