Oxford English Dictionary
I.1.a. A musical or vocal sound considered with reference to its quality, as sweet or harsh, loud or soft, clear or dull.
5.a. A particular quality, pitch, modulation, or inflexion of the voice expressing or indicating affirmation, interrogation, hesitation, decision, or some feeling or emotion; vocal expression.
d. transf. A particular style in discourse or writing, which expresses the person's sentiment or reveals his character; also spec. in literary criticism, an author's attitude to his subject matter or audience; the distinctive mood created by this.
Further definitions: the writer's or speaker's attitude toward material. Tone may be playful, formal, intimate, angry, serious, ironic, outraged, baffled, tender, serene, depressed, etc.
Tone differs from mood because it describes how the author or narrator feels about the characters, whereas mood describes how the reader feels when reading the story.
Tone manipulates the meaning of language, so that the same word may create a different meaning depending on the tone in which it is used.