BIP 65, outlined in BIP-65, introduced OP_CHECKLOCKTIMEVERIFY (CLTV). This opcode enforces that an output can only be spent after a specific block height or timestamp has been reached. It’s like a timed vault where you can’t open the door until the clock strikes a certain moment.

CLTV opened the door to more advanced scripting use cases, such as payment channels and trust-minimized escrow. By restricting spending until a future time, users can create contracts where funds remain locked until the agreed-upon conditions are met. Along with later proposals like OP_CHECKSEQUENCEVERIFY (BIP 112), CLTV helps developers build sophisticated layer-2 applications on top of Bitcoin.