What is the speaker's explanation of the raven's one response?

The speaker's explanation changes during the course of the poem -- from a rational explanation to a supernatural one.

At the beginning, the speaker's explanation is rational and conventional: He assumes that the raven was trained by its previous owner to voice this one word, and the utterance is not meaningful. It's merely a sound that the bird makes.

But midway through the poem, the speaker begins to invest the word with more meaning -- first as a reminder that someone else (the bird's original owner) must have been experiencing great unhappiness. Then the speaker begins to take seriously the idea that something supernatural is at work. The speaker speculates that the bird's responses are like the response of an oracle -- a meaningful sign transmitted by a supernatural being.

The speaker's shift in explanation happens in stages and parallels his changing mood. When he first sees the bird he smiles, and when the raven says "nevermore," the speaker "marveled" that the bird spoke the word so clearly ("plainly").

When the bird repeats the word  -- timed in such a way as to give the impression that the bird is once again responding to something the speaker said -- the speaker says he is startled. But he tells himself that the bird must be repeating the only word it knows -- learned from a human master who spoke the word frequently.

And notice that this is the first time the speaker invests the word with special meaning: he infers that the bird's previous owner must have experienced a great deal of unhappiness (to have spoken "nevermore" so often). The bird might not understand what the word means. But the bird is a little like an audio recorder, and the person who made the recording (the raven's owner) meant something sad or gloomy when he uttered the word.

Still smiling, the speaker sits and watches the raven, and begins to ask himself (playfully) if the raven itself intends a meaning ("Fancy upon fancy…"). And now we see the definitive shift of mood, and the speaker's thoughts of a supernatural explanation. The bird's "fiery eyes" have "burned" into the speaker's core, and the speaker starts to think he can sense an angel in the room.

"Then, methought, the air grew denser, perfumed from an unseen censer

Swung by Seraphim whose foot-falls tinkled on the tufted floor."

From this point on, he speaks to raven with genuine emotion. He's worked up, as if the raven's answers really do mean something. And he quickly shifts from wondering if the raven is a benign messenger (from God) to thinking it is a "thing of evil."

The speaker's responses to the raven are no longer those of an amused, composed person who accepts a rational explanation for the bird's utterances. Now he sees the raven's "nevermore" as a series of meaningful denials -- first that the speaker will experience respite from his sad memories, next that Lenore exists in some distant paradise, and last that the raven will finally leave the speaker alone.