Tonic

(ton'ik.) (From Gr. to nos, a sound.) (Mus.) The principal note of the key; upon it all the regular melodies depend; and with it all of them terminate, so far as the chief melody is concerned, the inner parts of the harmony concluding on the third or mediant, and the fifth or dominant. The name is applied also to the octaves above and below the key-note.-(Med.) One of a cla of medicines that increase the tone of the muscular fibre, and give vigor and action to the system. Preparations of iron and quinine are among the best known tonics. [s. 900]