Oxford English Dictionary
1.a. A genre comprising any of the types of melodramatic work, esp. exciting by exaggeration and sensationalism, and often (chiefly in earlier use) accompanied by music appropriate to the action; the style of drama characteristic of such a piece.
1.b. Originally: a stage play, usually romantic and sensational in plot, and interspersed with songs, in which the action is accompanied by orchestral music appropriate to the various situations (now hist.).
Later (as the musical element ceased to be regarded as essential): a play, film, or other dramatic piece characterized by exaggerated characters and a sensational plot intended to appeal to the emotions.
Wikipedia: "The term melodrama is most often used pejoratively [disparagingly] to suggest that the work the term is applied to lacks sophistication or subtlety."
General qualities of melodrama:
stock plot devices: