An inertial observer sits at the origin of his rest frame.
The points in 4D Minkowski space that have an (invariant) 4D spacetime
**interval of zero,
from the observer form a 4D cone**.
is a lightlike separation, that is, the cone consists of the points in the future that can be reached by light emitted by the
observer plus the points in the past from which light could be reaching the observer.
(The 4D cone is basically a spatial 3D sphere for every time. The size of the sphere grows with the time difference from the present.)
This is referred to as the
**light cone**.
Often this light cone is pictured in the time dimension plus two space dimensions to make it understandable.

Jim Branson 2012-10-21