Transaction or output restrictions that permit spending only after a certain point in time. They are valuable for postdating transactions and securing funds to a specific date in the future.
Bitcoin uses timelocks to introduce time-based transaction conditions, enabling various use cases such as smart contracts and off-chain protocols like the Lightning Network.
Timelocks have been present in Bitcoin since its genesis and can be implemented using the nLocktime field in a transaction. Other examples of current timelocks in Bitcoin include CHECKLOCKTIMEVERIFY (CLTV) and CHECKSEQUENCEVERIFY (CSV).
Timelocks are also a building block in the various covenant proposals under discussion, such as CHECKOUTPUTVERIFY, SIGHASH_ANYPREVOUT, OP_TAPLEAFUPDATEVERIFY, OP_CAT, and others.