One-way platform
A platform you can jump up through from below but land on from above — collision in one direction only.
Physically impossible, mechanically indispensable: the one-way platform lets a player rise through the floor above without hitting it, yet still stands there to be landed on. It is built by enabling collision only when the character is above the platform's top edge and moving downward; in every other case the platform is passable.
Many games add a gesture to drop through on purpose — hold down and jump. The trick appeared as far back as the arcade Mario Bros. and Bubble Bobble, and is now a checkbox on nearly every platformer engine. It is a special case of collision filtering rather than real geometry.