Definition: Clipless pedals are bicycle pedals that have a special mechanism that attaches you foot to the pedal using a special cleat mounted to the bottom of your shoe. This type of pedal creates a very efficient connection between you and your bike.
Clipless pedals are about the most efficient way to attach your foot to the pedal. The rider steps into the mechanism and can then apply pedaling forces in the upward,downward, forward and rearward directions. To release from the pedal, the rider simply needs to twist their foot.
The term clipless is a bit confusing, since many would describe the mechanism in a clipless pedal as a clip. However, they are known as clipless because they are an alternative to the original toeclip pedals that use straps and a toe cage to hold your foot to the pedal.
Examples: I got stuck in my clipless pedals and fell over.