A crossover is when two different continuities meet, typically belonging to different franchises. In canon, they tend to be extremely rare because it means merging two or more shows and their respective universes, but are an extremely popular subject of fanon.
Kim Possible has officially been crossed over with Lilo and Stich, though the nature of this episode's canonicity is highly debated. Kim Possible has a large number of crossovers in both fanfiction and fan art, viewable at Fanfiction.net and DeviantART
There are a few types of crossovers. Fusions, which combine two franchises and their histories/back stories, and Alternate Universes which typically place a Kim Possible character into another franchise's universe (or vice versa).