My comment is brief, but I want to make it anyway.

"Nevermore" is the central word of the poem, if we go by Edgar Allan Poe's 1846 essay, "The Philosophy of Composition." In this essay he writes about how he wrote the poem, and it all started -- according to his essay -- with that one word. "The Philosophy of Composition" is a short essay, well worth reading (see the link below). In the essay, you'll find a discussion of how Poe decided on using a raven (at first, he was thinking of a human and then of a parrot) and of how he arranged the questions from the speaker from the general to the most personal, always receiving the same, one-word answer.

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.

In general, the word mean "never" or "never again."  But the meaning is slightly different at different points in the poem.

The first time it appears, it means "never."  The speakers asks the raven to tell him its name and it says it will not.

It means the same thing the third time it appears -- bird says he won't leave.

But then in Stanza 13, there's a different meaning.  Now it means "never again."  The speaker is saying that Lenore will never again sit in that chair.  Most of the rest of the times the word appears in the poem, that is what it means -- including the last line, which says that the speakers soul will never again be lifted out of shadow.