Melancholia is a deep sadness with little energy — a heavy, slow feeling that sits on you. It is an old, rather bookish word. Ancient doctors believed the body had four fluids, called humours; too much black bile was thought to make this mood, and the Greek name literally means blackness of the bile. We no longer explain feelings that way, but the word stayed in literature, art, and the history of medicine. Everyday English now says sadness, feeling down, or depression when it is an illness. Melancholia sounds formal, old, and a little medical. It is a twin of melancholy — same source, with melancholia often the more antique or clinical twin. A poem or a museum wall might use it; a text to a friend almost never would.