World line

The world line of a particle is the path that it traces out in 4-dimensional spacetime. 

It is not an orbit or a trajectory. This is because of the the inclusion of the time dimension. 

The idea was actually pioneered by Herman Minkowski and the term is used often in the special and general theories of relativity by Albert Einstein.

The world line is the sequence of spacetime events that correspond to the history of the particle. It is a time-like curve in spacetime. 

The path of a particle through spacetime can be considered a succession of events. This series of events can be linked together to form a line. This line represents a particles progress through spacetime.