Anime Crossover - this occurence when elements of two or more animes like characters and characteristics
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.

Example :

To Aru No Railgun Crossover

Kannagi Hinamori

Spice Wolf Amu Horo

Rozen Maiden Amulet

K-On Clover

 

Utau Hatsune