Chuck Cunningham syndrome is a term of criticism, applied when an important character in a television series is removed with little or no explanation.
The term derives from the Chuck Cunningham character in the American series Happy Days. Chuck, the oldest of the three children in the Cunningham family, initially appeared in the episode “Love and the Happy Days” (in the show Love, American Style), which served as the pilot for what became the Happy Days series. However, in Happy Days Chuck appeared as a superfluous character (usually on his way to basketball practice). He was written out of the series at the beginning of the second season with the explanation that he was going to college. After the second season, he was never mentioned again; episodes referred to the Cunninghams as having two, rather than three, children.”
I rather enjoyed skimming through the list of shows tied into this listing.