The boundary of the region from which light signals can reach an observer is the light cone. In spacetime, it represents all events connected to the observer by light rays traveling at the universal speed c.
For an observer at a particular event, the past light cone contains signals that could already have arrived, while the future light cone contains events the observer could influence with a light signal. The cone’s surface itself is “lightlike”; its interior contains causally connected events involving slower-than-light motion.
Light cones became central to Hermann Minkowski’s spacetime formulation of special relativity, developed after Einstein’s 1905 theory. They replace the idea of separate absolute space and time with a geometric picture of cause and effect.
A common mix-up is the event horizon of a black hole. An event horizon is a one-way causal boundary, whereas a light cone is the local spacetime structure around every event, whether or not gravity is strong.