Cas

A nickname for Castiel, first used by Dean in 4.04 Metamorphosis and subsequently used by Sam and Anna as well. Few instances of its use in fandom before this, subsequently its use became widepread.

The shows' writers spell the nickname "Cass" (Source). In fandom, and particularly in Fanfiction it is spelled "Cas.

On the Season 4 DVD the english subtitles call him "Cass" and other times it's "Cas".