What does "tonic" mean?

Additional senses

1. 2.(Med.) Increasing strength, or the tone of the animal system; obviating the effects of debility, and restoring heatly functions. Tononic spasm. (Med.) See the Note under Spasm.
2. 3.(Phon.) A tonic element or letter; a vowel or a diphthong.
3. 4.(Mus.) The key tone, or first tone of any scale.
4. 5.(Med.) A medicine that increases the srength, and gives vigor of action to the system. Tonic sol-fa (Mus.), the name of the most popular among letter systems of notation (at least in England), based on key relationship, and hence called "tonic." Instead of the five lines, clefs, signature, etc., of the usual notation, it employs letters and the syllables do, re, mi, etc., variously modified, with other simple signs of duration, of upper or lower octave, etc. See Sol-fa.