These crossovers often form a "universe" or general continuity. These are often given names such as the "Buffyverse" or "Whoniverse" for their respective collections. Given that, consider that if every crossover was mapped out, the whole connected group could be considered, in fan terms, to be in the same universe. A group of fans have created such a map, and here what such a map looks like:
St. Elsewhere. In the final moments of the last episode, it was revealed that the entire run of the show was the dream of an autistic child named Tommy Westphall. The ending is a cop-out from an otherwise well-written series, but the implications were thus:
If "St Elsewhere" exists only within Tommy Westphall's mind, then so does every other series set within the same fictional sphere.Once fans started tracking the numerous crossovers and spin-offs from St Elsewhere, they created a map of 282 shows, all connecting to the intersection of "St Elsewhere and Homicide," and dubbing this phenomenon the "Tommy Westphall Universe."
We are through the looking glass here people.