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).

More examples can be found in the various proposals and discussions related to covenants, such as CHECKOUTPUTVERIFY, SIGHASH_ANYPREVOUT, OP_TAPLEAFUPDATEVERIFY, OP_CAT, and others.