1. The first degree of the scale; the keynote.
2. A tonic syllable or accent.
3. Using differences in tone or pitch to distinguish between words that are otherwise phonemically identical; a tonic language.
4. Pertaining to tone or accent in speech.
5. Bearing the principal stress or accent; usually accompanied by a change in pitch.
6. A reference to a tone or tones.
7. Pertaining to or founded on the keynote, or first tone, of a musical scale: a tonic chord.

A reference to the study of tones (intonation) in speech.