Definition of tonic

• a. - Of or relating to tones or sounds; specifically (Phon.), applied to, or distingshing, a speech sound made with tone unmixed and undimmed by obstruction, such sounds, namely, the vowels and diphthongs, being so called by Dr. James Rush (1833) " from their forming the purest and most plastic material of intonation."
• a. - Of or pertaining to tension; increasing tension; hence, increasing strength; as, tonic power.
• a. - Increasing strength, or the tone of the animal system; obviating the effects of debility, and restoring healthy functions.
• n. - A tonic element or letter; a vowel or a diphthong.
• n. - The key tone, or first tone of any scale.
• n. - A medicine that increases the strength, and gives vigor of action to the system.