What is the meaning of "nevermore" repeated by the raven? How does it change throughout the poem?

The brilliance
of Poe's immortal poem "" is this: an ordinary corvid flies into the narrator's house
one night, perches over his door, and starts squawking (as birds are wont to do). Due to the
extremely fragile state of the narrator's tortured mind, this simple, simple soundrepeated over
and overcauses him to go hopelessly mad. 

"Nevermore" is the sound
that the narrator hears when the raven opens its mouth. It's no great surprise that his mind
created something unusualafter all, we hear the words "cock-a-doodle-doo" from
roosters. The word does not immediately cause stress upon his brain, just as the melodic chimes
begin sounding beautifully in Poe's "" before they turn to gradually to a cacophony of
torture. 

In both poems, a lovesick man hears a neutral external noise and is
driven mad by his own mind's inability to find peace in its agony. "Nevermore" becomes
a cruel mantra; it becomes the jeering of the gods at his own attempt to mourn his lost love; it
becomes the beak in his heart and the shadow upon his soul.