The most common definition has Slash as a Fan Fic that pairs two (or more) characters of the same sex (a manifestation of the LGBT Fanbase).
There are a few other definitions around:
Originally, the term "Slash" was more specific, referring only to sexually explicit homoerotic Fan Fic (even more specifically, to male homosexual experiences).
Some say that fanfics about characters who are gay [...] [tvtropes.org]