What BIP number introduced SegWit?

BIP 141 introduced Segregated Witness, commonly called SegWit.

BIP 141, formally titled “Segregated Witness (Consensus layer),” defined a change to Bitcoin’s transaction format. It separated signature data, called witness data, from the transaction information used in the traditional transaction hash and block structure.

SegWit addressed transaction malleability, a problem in which a transaction’s identifying hash could be altered without changing its essential meaning. Its design also changed how block capacity was measured, allowing more effective transaction space without a conventional hard fork.

SegWit activated on Bitcoin in August 2017 at block height 481,824. The nearby BIP numbers are easy to confuse: BIP 143 specified signature-verification changes, BIP 144 covered peer services, and BIP 142 proposed an address format that was later withdrawn. SegWit should also not be confused with the separate, abandoned SegWit2x proposal.