The curve that represents the history of a particle in spacetime is called its worldline. Every point on the curve represents one event: the particle’s position at a particular time. Together, those points trace the particle’s complete passage through four-dimensional spacetime.
The idea became especially useful after Hermann Minkowski’s 1908 geometric formulation of special relativity. In a spacetime diagram, time is plotted alongside one or more spatial dimensions. A stationary object has a straight, nearly vertical worldline, while a moving object has a tilted one. Acceleration makes the line curve.
A worldline is not the same as an ordinary orbit or trajectory. An orbit records position through space, such as Earth’s path around the Sun. Earth’s worldline also includes time, so it forms a helix-like path through spacetime rather than a closed spatial circle.
For a freely falling massive particle, the worldline is a timelike geodesic. A photon follows a null worldline and lies along the boundary of a light cone. Proper time can be used as a parameter along a massive particle’s worldline.