A rule on coins that says they can't be spent until a certain date or block. It's used for inheritance plans, savings, and keeping Lightning payments safe.
Go deeperThere are two kinds. An absolute timelock (BIP 65) means "not before block X or date Y." A relative timelock (BIP 112) means "not until N blocks after these coins confirmed." Lightning uses them to give you time to stop a cheater, and some inheritance setups use a backup key that only works after a year. A mistake can lock up your own coins, so stick to well-tested wallets.