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A timelock is a type of encumbrance that restricts bitcoin from being spent until a specified time in the future or block height is met. One can think of a timelock as a timer for bitcoin transactions.

Timelocks are used in both payment channels and HTLCs, but they are also a simple and elegant way to schedule a transaction to occur on a certain date (CLTV) or block height (CSV) in the future.

CLTV (CheckLockTimeVerify)

CLTV is a soft fork in BIP65 that allows timelocks to act as an encumbrance on transaction outputs. In this way, one can set up a transaction to process only after or before a specified date. 

CSV (CheckSequenceVerify)

CSV is part of the BIP68/112/113 soft fork that allows inputs in valid blocks only if it’s relative lock time expires before it is added to the block. CSV is a critical part of Lightning Network transactions.