Hashed Time-Locked Contract transactions.

Set out a script that pays one person if they reveal a secret, or refunds the sender after a deadline.

This is the hashed time-locked contract, or HTLC. A script is a small program that decides who can spend coins. Here the seller can claim the coins by revealing a secret, and if they don't, the buyer gets them back once time runs out. The BIP notes that the Lightning Network uses such scripts to pass payments along safely.