We do not currently know of any synonyms for null infinity.
The noun null infinity is defined as:
In a closedspace-time, especially for an Einstein cylinder, null infinity for a given point is the limit of convergence of the light cone most distant from the given point. That is, it is the limit of a set of points that each have a time-likeseparation from every point that has a space-likeseparation from the given point.