tonic

Tonic is a term used in several fields, most notably music, beverages, medicine, and biology. In music theory, the tonic is the first degree of a diatonic scale and the root of the corresponding tonic triad. It serves as the principal pitch center around which melodies and harmonies revolve, and most phrases tend to resolve to the tonic through cadences such as the authentic cadence. The key or tonic of a piece helps establish its sense of home.

In Western tonal music, the tonic defines the key and provides a sense of rest and resolution. The tonic is contrasted with other scale degrees such as the dominant and subdominant, yet it remains the ultimate goal of many harmonic progressions that return to it.

Tonic water is a carbonated beverage flavored with quinine, a bitter compound historically used to prevent malaria. The bitterness of quinine gave tonic water its characteristic taste, which was intensified with sugar in early formulations. Modern tonic water contains much smaller amounts of quinine and is often sweetened and citrus-flavored. It is commonly mixed with gin to form the drink known as a gin and tonic.

In medicine and everyday language, a tonic is something that restores strength or vitality, such as a tonic medicine or tonic herbs. In physiology, tonic refers to sustained activity or tone, for example tonic muscle contraction or tonic receptors that respond continuously to stimulation, as opposed to phasic responses.