trilogy

[ tril-uh-jee ]
/ ˈtrɪl ə dʒi /

noun, plural tril·o·gies.

a series or group of three plays, novels, operas, etc., that, although individually complete, are closely related in theme, sequence, or the like.
(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.
a group of three related things.

Origin of trilogy

From the Greek word trilogía, dating back to 1655–65. See tri-, -logy
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

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British Dictionary definitions for trilogy

trilogy

/ (ˈtrɪlədʒɪ) /

noun plural -gies

a series of three related works, esp in literature, etc
(in ancient Greece) a series of three tragedies performed together at the Dionysian festivals

Word Origin for trilogy

C19: from Greek trilogia; see tri-, -logy
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012