The trajectory followed by an object in spacetime. The fact that spacetime is four-dimensional makes world lines difficult to visualize but, if the Universe had only one dimension in space and one in time, the world line could be drawn on a graph with time plotted vertically and distance plotted horizontally. A particle at rest with respect to the coordinate system would have a world line which runs along the vertical axis, while moving particles would have world lines which are curves or straight lines sloping upwards. In the real Universe, the path of a moving particle is a curved line in spacetime.
Subjects: Astronomy and Astrophysics — Physics.