A crossover is the placement of two or more otherwise discrete fictional characters, settings, or universes into the context of a single story. They can arise from legal agreements between the relevant copyright holders, common corporate ownership or unofficial efforts by fans.
Comic book crossovers may be traced back to the Golden Age, where characters frequently teamed up on the cover (though far more rarely on the inside). Speed Comics number 32, artwork by Alex Schomburg.
An early example of the comics crossover: Captain Marvel and Bulletman join forces to battle Captain Nazi. Master Comics number 21, artwork by Mac Rayboy.
The Mysterious Island novel is a crossover sequel to Verne's famous Twenty Thousand Leagues Under the Seas (1870) and In Search of the Castaways (1867–68).
Allan Quatermain
The Golden Girls is an American sitcom created by Susan Harris that aired on NBC from September 14, 1985, to May 9, 1992, with a total of 180 half-hour episodes, spanning seven seasons. With an ensemble cast starring Bea Arthur, Betty White, Rue McClanahan, and Estelle Getty, the show is about four older women who share a home in Miami, Florida. It was produced by Witt/Thomas/Harris Productions, in association with Touchstone Television. Paul Junger Witt, Tony Thomas, and Harris served as the original executive producers.
Rue McClanahan in 2007
Estelle Getty at the 41st annual Primetime Emmy Awards in 1989