Melancholia or melancholy (Ancient Greek: μελαγχολία, romanized: melancholía; from μέλαινα χολή, mélaina cholḗ, '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.^{[citation needed]}

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. In the 19th century, melancholia was considered to be rooted in subjective 'passions' that seemingly caused disordered mood (in contrast to modern biomedical explanations for mood disorders). In Victorian Britain, the notion of melancholia as a disease evolved as it became increasingly classifiable and diagnosable with a set list of symptoms that contributed to a biomedical model for the understanding mental disease. 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.

Related terms used in historical medicine include lugubriousness (from Latin lugere, 'to mourn'), moroseness (from Latin morosus, 'self-will or fastidious habit'), wistfulness (from a blend of wishful and the obsolete English wistly, meaning 'intently'), and saturnineness (from Latin Saturninus, 'of the planet Saturn').

The name "melancholia" comes from the old medical belief of the four humours: disease or ailment being caused by an imbalance in one or more of the four basic bodily liquids, or humours. Personality types were similarly determined by the dominant humor in a particular person. According to Hippocrates and subsequent tradition, melancholia was caused by an excess of black bile, hence the name, which means "black bile", from Ancient Greek μέλας (melas), "dark, black", and χολή (kholé), "bile"; a person whose constitution tended to have a preponderance of black bile had a melancholic disposition. In the complex elaboration of humorist theory, it was associated with the earth from the Four Elements, the season of autumn, the spleen as the originating organ and cold and dry as related qualities. In astrology it showed the influence of Saturn, hence the related adjective saturnine.

Melancholia was described as a distinct disease with particular mental and physical symptoms in the 5th and 4th centuries BC. Hippocrates, in his Aphorisms, characterized all "fears and despondencies, if they last a long time" as being symptomatic of melancholia. Other symptoms mentioned by Hippocrates include: poor appetite, abulia, sleeplessness, irritability, agitation. The Hippocratic clinical description of melancholia shows significant overlaps with contemporary nosography of depressive syndromes (6 symptoms out of the 9 included in DSM diagnostic criteria for a Major Depressive).

In ancient Rome, Galen added "fixed delusions" to the set of symptoms listed by Hippocrates. Galen also believed that melancholia caused cancer. Aretaeus of Cappadocia, in turn, believed that melancholia involved both a state of anguish, and a delusion. In the 10th century Persian physician Al-Akhawayni Bokhari described melancholia as a chronic illness caused by the impact of black bile on the brain. He described melancholia's initial clinical manifestations as "suffering from an unexplained fear, inability to answer questions or providing false answers, self-laughing and self-crying and speaking meaninglessly, yet with no fever."