A set of three books, films, plays or games that belong together — same world, same people, or one story told across three parts. The Lord of the Rings is a trilogy; so is the first Star Wars set. The word says something more than 'three': it says the three were meant to go together, and that reading or watching all three gives you something one alone cannot.

Though actually, the writer does not always plan it. Plenty of trilogies happen by accident — a book sells well, a second follows, a third finishes it off, and the publisher prints 'The X Trilogy' on the box afterwards. And once a fourth part arrives, English has no comfortable word for it, so people either keep saying trilogy out of habit or joke about it.

It is a countable noun: a trilogy, two trilogies (note the y turns to ies). You say a trilogy of novels, or the Godfather trilogy. The word came into English from Greek theatre, where three tragedies were performed one after another at a festival, and it still carries a slightly literary, respectful tone — you would not usually call three random sequels a trilogy unless they really hang together.