Melancholia (from Greek μελαγχολία - melancholia, "sadness", literally black bile), also lugubriousness, from the Latin lugere, to mourn; moroseness, from the Latin morosus, self-willed, fastidious habit; wistfulness, from old English wist: intent, or saturnine, (see Saturn), in contemporary usage, is a mood disorder of non-specific depression, characterized by low levels of both enthusiasm and eagerness for activity.

In a modern context, "melancholy" applies only to the mental or emotional symptoms of depression or despondency; historically, "melancholia" could be physical as well as mental, and melancholic conditions were classified as such by their common cause rather than by their properties.

Similarly, melancholia in ancient usage also encompassed mental disorders which might now be classed as schizophrenias or bipolar disorders.