fused up in one anime. It's usually existed in comedy type of animes.
There are two types of anime crossovers:
1. Cameo(Guest) Appearances - when one or more characters from another anime are suddenly appeared on
the anime you're currently watching.
2. Parody - when the characters from the anime you currently watching portrays the characteristics of another
characters from another animes. A character from the anime you currently watching are cosplaying
a character from another anime can be a good example.
To Aru No Railgun Crossover
Spice Wolf Amu Horo
Rozen Maiden Amulet