nLocktime

nLockTime is the protocol-level name for Bitcoin's locktime field - a 32-bit value in every transaction specifying the earliest block height or Unix timestamp at which the transaction may be included in a block.

The n prefix is a holdover from Satoshi's original C++ code, where integer fields conventionally started with n. You'll see it in source code, BIPs, and protocol docs; in conversation, people just say "locktime."

Value semantics:

• 0 - immediate validity (default).
• 1 to 499,999,999 - interpreted as a block height.
• 500,000,000 and above - interpreted as a Unix timestamp (seconds since the epoch).

nLockTime is paired with the per-input nSequence field. If every input's nSequence is 0xffffffff, nLockTime is ignored - the transaction is treated as having no time lock regardless of the field's value. Setting any nSequence below the maximum makes nLockTime enforceable.

This little quirk is the foundation for Replace-by-Fee, which uses nSequence to signal "this transaction can be replaced." It's also how older relative-locktime workflows handled things before CHECKSEQUENCEVERIFY was added.

See Locktime for the practical view and Absolute Locktime for the contrast with relative-locktime via nSequence.