This definition comes from the technical glossary.
Part of a transaction which indicates the earliest time or earliest block when that transaction may be added to the block chain.
Locktime — Bitcoin.org Developer Guide
nLockTime — Bitcoin Wiki
How is locktime enforced in Bitcoin Core? — Bitcoin StackExchange
Bitcoin.org is community supported: 1GwV7fPX97hmavc6iNrUZUogmjpLPrPFoE