<p><strong>Noun:</strong></p><ul><li>a series or group of three plays, novels, operas, etc., that, although individually complete, are closely related in theme, sequence, or the like.</li><li>(in ancient Greek drama) a series of three complete and usually related tragedies performed at the festival of Dionysus and forming a tetralogy with the satyr play.</li><li>a group of three related things.</li></ul>