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A series of two related works, many times being relevant to movies, books, plays, etc.
Bicycle = Bike with 2 wheels
Tricycle = Bike with 3 wheels

Bilogy = Story told through 2 movies
Trilogy = Story told through 3 movies

"Kill Bill is my favorite bilogy... mostly because I can't think of any others."
by BobbyGo August 04, 2012
A series with more than three books
Harry Potter is a bilogy
by Courtney and Jackie December 29, 2007