NLockTime
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nLockTime is a parameter of a transaction, that, if any input indicates so (by having nSequence not equal to UINT_MAX), mandates a minimal time (specified in either unix time or block height), before which the transaction cannot be accepted into a block. If all inputs in a transaction have nSequence equal to UINT_MAX, then nLockTime is ignored.
See lock_time in the protocol specification
See Also on BitcoinWiki[edit]