Cas
Cas is a nickname for Castiel, first used by Dean in 4.04 Metamorphosis and from Season 5 commonly used by other characters.
Before its use in canon there were relatively few instances of the use of this diminutive in fandom, and when it did appear it was generally pilloried. Subsequently its use became widespread.
The shows' writers spell the nickname "Cass" in scripts source, and it also appears that way in Tie-in novels and the Official Companion books. It is spelled Cass in the Season 7 promo and the promo for the sixth episode of Season 8.
On the Season 4 DVD the English subtitles call him "Cass" and other times it's "Cas".
In fandom, and particularly in Fanfiction it is almost exclusively spelled "Cas". Fans often get annoyed at the Show's writers for spelling it wrong.
One theory as to the spelling "Cass" is that the writers based Castiel on the angel Cassiel, best known in popular culture from the movies Wings of Desire and the American remake City Of Angels.
