CheckLockTimeVerify (CLTV)

OP_CHECKLOCKTIMEVERIFY (CLTV) is the Bitcoin Script opcode that enforces absolute locktime inside a script. It's the script-level companion to the transaction-level nLockTime field.

The opcode's semantics: when CLTV executes, it checks that the spending transaction's nLockTime is at least a specific value (which CLTV reads from the stack). If that condition holds, the script continues; if not, the spending fails. So a locking script can require "to spend, the spending transaction must claim a locktime ≥ X" - effectively locking the output until that block height or timestamp.

Introduced via BIP-65 in December 2015, CLTV is one of the script primitives that made advanced Bitcoin constructions practical. Common uses:

• Payment channels - close-out scripts include CLTV-based delays so disputes can be resolved before unilateral spending.
• HTLCs - the time-based fallback ("if the preimage isn't revealed by block N, the sender can reclaim") uses CLTV.
• Atomic swaps - timeout refunds on cross-chain trades.
• Vault constructions - mandatory withdrawal delays for cold-storage security.
• Inheritance scripts - heir can claim funds after a long inactivity period.

CLTV was deployed by replacing the previously-disabled OP_NOP2 opcode. This is the standard Bitcoin pattern for adding new functionality via soft fork: take a NOP that does nothing, redefine it to do something useful. Older nodes that don't recognize the new meaning just see OP_NOP2 doing nothing - the script still succeeds for them, so the new rules are strictly additional restrictions that old nodes treat as "valid" by default.

Companion to OP_CHECKSEQUENCEVERIFY (CSV), which provides relative locktimes via BIP-68/112. Together they're the time-based scripting toolkit that makes Lightning and many other layer-2 designs possible.