Melancholia or melancholy (from Greek: µέλαινα χολή melaina chole, (meaning black bile) is a concept found throughout ancient, medieval, and premodern medicine in Europe that describes a condition characterized by markedly depressed mood, bodily complaints, and sometimes hallucinations and delusions.

Melancholy was regarded as one of the four temperaments matching the four humours. Until the 18th century, doctors and other scholars classified melancholic conditions as such by their perceived common cause – an excess of a notional fluid known as "black bile", which was commonly linked to the spleen. Hippocrates and other ancient physicians described melancholia as a distinct disease with mental and physical symptoms, including persistent fears and despondencies, poor appetite, abulia, sleeplessness, irritability, and agitation. Later, fixed delusions were added by Galen and other physicians to the list of symptoms. In the Middle Ages, the understanding of melancholia shifted to a religious perspective, with sadness seen as a vice and demonic possession, rather than somatic causes, as a potential cause of the disease.

During the late 16th and early 17th centuries, a cultural and literary cult of melancholia emerged in England, linked to Neoplatonist and humanist Marsilio Ficino's transformation of melancholia from a sign of vice into a mark of genius. This fashionable melancholy became a prominent theme in literature, art, and music of the era.

Between the late 18th and late 19th centuries, melancholia was a common medical diagnosis. In this period, the focus was on the abnormal beliefs associated with the disorder, rather than depression and affective symptoms. However, in the 20th century, the focus again shifted, and the term became used essentially as a synonym for depression. Indeed, modern concepts of depression as a mood disorder eventually arose from this historical context. Today, the term "melancholia" and "melancholic" are still used in medical diagnostic classification, such as in ICD-11 and DSM-5, to specify certain features that may be present in major depression.