What does the phrase "nevermore" mean in "The Raven"?

In Poe's famous poem "The Raven," the speaker, who has just lost his true love, Lenore, slowly goes mad from grief.  The raven seems to represent a visitor from the world of the dead, and the only phrase it utters, "nevermore," changes through the course of the poem.

At first, the raven gives it as a name, causing the speaker to marvel at such a strange creature and wonder about its previous owner.  Then, the word reminds the speaker that Lenore will "nevermore" be with him, and he begins to become enraged.  He asks the raven if Lenore is in heaven, and again, it answers, "nevermore."  In the end, the speaker goes insane, and the word "nevermore" can mean here that he will never be sane again.