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.

Between the late 18th and late 19th centuries, melancholia was a common medical diagnosis, and modern concepts of depression as a mood disorder eventually arose from this historical context.

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 term "melancholia" originated from the ancient medical belief of the four humours, with melancholia being caused by an excess of black bile. 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. In ancient Rome, Galen added "fixed delusions" to the list of symptoms and believed that melancholia could cause cancer.

The 10th-century Persian physician Al-Akhawayni Bokhari described melancholia as a chronic illness caused by the impact of black bile on the brain, with symptoms such as unexplained fear and inability to answer questions. In the Middle Ages, the understanding of melancholia shifted to a religious perspective, with sadness seen as a vice and demonic possession as a potential cause of the disease.

Painters like Albrecht Dürer depicted melancholia in their work, with Dürer's famous engraving "Melencolia I" interpreted as a portrayal of waiting for inspiration. Robert Burton's "The Anatomy of Melancholy" (1621) provided an extensive analysis of the subject from both literary and medical perspectives, suggesting music and dance as critical treatments for mental illness. The Encyclopédie of Diderot and d'Alembert also noted that the causes of melancholia were similar to those of mania, including grief, pain, passions, and unsatisfied love and sexual appetites. 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 into a mark of genius. This fashionable melancholy became a prominent theme in literature, art, and music of the era. Notable literary works include Robert Burton's "The Anatomy of Melancholy" (1621), Sir Thomas Browne's "Religio Medici" (1643), and Edward Young's "Night-Thoughts" (1742-1745). In the visual arts, intellectual melancholy was often depicted in portraiture, while in music, it was associated with composer John Dowland. Shakespeare's Prince Hamlet epitomized the melancholic man.

From the 18th to 19th centuries, melancholia became more about abnormal beliefs, losing its connection to depression and affective symptoms. However, in the 20th century, the term returned as a synonym for depression. Sigmund Freud, Gordon Parker, Dusan Hadzi-Pavlovic, Michael Alan Taylor, and Max Fink all contributed to defining melancholia in various ways. 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.