The wor def is hip hop (including graffiti) slang for 'great', 'ill', 'dope', 'fly': generally, a positive adjective used as a superlative. Taken from the root word 'definitive', if something is 'def' then it is classified as being very good. If something is very definitive in its representation of its genre or classification, it is 'def'. Def Jam records took their name from this term.