A series with more than three books
Harry Potter is a bilogy
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."