Emily Dickinson Lexicon

TON'IC, a. [from Gr. τονος, L. tonus. See Tone.]

1. Literally, increasing tension; hence, increasing strength, as tonic power.
2. In medicine, increasing strength, or the tone of the animal system; obviating the effects of debility, and restoring healthy functions.
3. Relating to tones or sounds.
4. Extended. [Not in use.] Brown. Tonic spasm, in medicine, is a steady and continuous spastic contraction enduring for a comparatively long time. It is opposed to a clonic spasm, in which the muscular fibers contract and relax alternately in very quick succession, producing the appearance of agitation. In tonic spasms, however, there is always alternate contraction and relaxation. The spasms of tetanus are tonic.