The theory that holds that there are four basic temperaments, i.e. melancholic (gloomy), choleric (irritable), sanguine (cheerful), and phlegmatic (apathetic) that are determined by the predominance of one of the four humors, i.e. black bile, yellow bile, phlegm, and blood, respectively, to characterize a person's psychological state.
Its application in medicine is now no longer as extensive as before. Nonetheless, it is still used nowadays by psychologists and the lay public.
See also: humoralism.