A piece of work -usually in reference to literature- that was written by the original author. Spin-offs, fan fiction, and any work not written by the original author of that fictional universe is considered non-canon.
I got sad at the end of Rurouni Kenshin: Seisouhen. Good thing it took out a lot of chunks from the original plot and wasn't written by Nobuhiro Watsuki, or else it would have been considered canon. :D
Oh my GOD that Naruto fan fiction SUCKED! Thank God it wasn't canon.