I gave my definition on a different thread.
I believe that defining it to mean that ALL popes after Pius XII were illegitimate is too narrow.
I believe that applying the term to people who simply acknowledge the fact of a normal interregnum is too broad.
Nor do I believe that any specific reason should be required, i.e. whether he's a heretic, illegitimately elected, impeded by Benny or Siri, etc. Any reason other than that the Pope died and we're waiting for a new election should qualify.
So I propose something along the lines of ... a theological position holding that the man widely believed to be the legitimate Pope is not in fact the legitimate Pope, and that the See is empty.
We could drop the "and the See is empty" part, to include people like, say, those who believe there's an alternative candidate, such as Michael I (while he was alive) or Pius XIII. I think those can be rightly called SVs in that they held their various conclaves / elections precisely due to the fact that they were SVs in the first place, believing that the See was empty.
So, this would in fact consider the Bennyvacatists to be sedevacantists, and they truly are, despite their emotional denials. Bergoglio is widely considered to be the pope, but they reject his legitimacy and hold the See to be empty.
It would exclude the sedeprivationists, strictly speaking, or the sede-impoundists (Father Chazal).