The light cone of an event

Similarly, the light spreading out from an event forms a (3D) cone in (the 4D) space-time. This cone is called the future light cone of the event. In the same way we can draw another cone, the past light cone, which is the set of events from which a pulse of light is able to reach the given event. 

Together, the past and future light cones divide the space time into four regions:

- The timelike region

- The lightlike region

- The spacelike region

- The absolute past and future