World line
A world line is the unique path that an object has as it travels through both space and time, usually called spacetime. As we learn from special relativity, the faster an object goes, the more time slows down for that object.  As you can see in the illustration to the right, the slower object has a quicker passage of time than the very fast object, the one which time passes much more slowly for. When an object reaches the speed of light, it will be zero on the t axis, meaning that it will have made no progress in the time direction. Basically, the world lines show that when the speed of light is reached, time stops for the observer. World lines are very often used in theoretical physics and special relativity, as well as general relativity.