A 32-bit field in a transaction that means either a block height at which the transaction becomes valid, or a UNIX timestamp. “Zero” means the transaction is valid in any block. A number less than 500000000 is intrepreted as a block number (the limit will be hit after year 11000), otherwise a timestamp.
Malone, J.A (2015). Glossary of Bitcoin Terms and Definitions. United States: Lulu Press, Inc