A group of five dramatic or literary works related in subject or theme.
Well-known quintets are hard to come by. Trilogies are considerably more popular in both the book and movie world. Movie quintets include:
However, the world's best-known quintet (not to be confused with "quintilogy", which, I'm reliably informed, isn't a word) is almost certainly The Hitch Hiker's Guide to the Galaxy by Douglas Adams.
They are a series of five novels, comprising:
For two years after Life, the Universe and Everything was published, the books were, indeed, a trilogy. Then So Long, and Thanks For All the Fish came out, "flatly contradicted everything up to and including itself", and because "quartet" sounds a bit high-and-mighty for comic sci-fi, the Hitch-Hiker series simply became the "Trilogy of Four". Copies of the books from this period have "Book One in the Trilogy of Four", "Book Two in the Trilogy of Four" etc. printed on their back covers. H2G2 remained a trilogy, and all remained well and good.
Eight years later, Mostly Harmless was published.
In keeping with the logic of the series, it was simply called "Book Five in the Trilogy of Four".
The Hitch-Hiker Trilogy is basically its own running gag. It's actually a pretty good gag, too, if you happen to get it. If not, there are plenty more inside the books themselves.