The cone-shaped diagram showing possible light paths from a spacetime event is a light cone.
In a spacetime diagram, the event sits at the cone’s apex. The future light cone contains events that could be reached by light or a slower signal; the past light cone contains events whose signals could reach the original event. The cone’s surface represents light moving exactly at the speed of light.
The diagram comes from special relativity’s merger of space and time. In a simplified diagram with one space axis and one time axis, light rays appear as diagonal lines; adding another space dimension turns those lines into a cone. The boundary separates timelike events, which can be causally connected, from spacelike events, which cannot influence one another without faster-than-light signaling.
A common mix-up is confusing a light cone with a Feynman diagram. A Feynman diagram is a calculation tool for particle interactions, whereas a light cone represents causal structure in spacetime. In general relativity, gravity can curve and distort light cones.