In the movie The Ninth Gate..

As the title states. What is the actual ninth gate supposed to be?

I think the fictional book in the film is about the "Nine Gates" of Hell or somesuch. So I guess it's the "last" one, the one that gains you entrance. Something like that. Not sure if the film's lore is particularly fleshed out.

Ninth gate is the last of the nine gates one needs to pass, in order to awaken Lucifer. At every gate you perform a ritual to pass it.