BIP 16 — Pay to Script Hash

BIP 16 (Pay to Script Hash) is a consensus soft fork that lets outputs commit to the hash of a redeem script, with the full script revealed only at spend time. It moved script complexity from sender to receiver, enabled practical multisig, and introduced addresses beginning with 3. It was activated by soft fork in 2012. Repository status: Deployed.

Debate relevance: Comes up in early-soft-fork history, in 'sender doesn't need to know your spending conditions' explanations, and in the historical OP_EVAL controversy (the competing proposal P2SH won out over). Common misstatements: Conflating P2SH with multisig — multisig is one common use of P2SH, not the same thing. Also, addresses starting with 3 are not necessarily multisig, and after BIP 49 many were single-key wrapped-SegWit.