It could be anything from simply an actor appearing on a different series as the character he or she plays on his or her own series to interfranchise
crossovers(all those times different Star Trek characters appear on each other shows or all those times Laverne & Shirley appear on Happy Days
or the characters from Happy Days appear on L & S,etc.) to two or more
different series combining in one mega storyline crossover.
Note:References of characters from other series can also be used but
only AS LONG AS THEY ARE TREATED AS REAL PEOPLE or CONSIDERED TO BE REAL PEOPLE in the context of the show.
