BIP 141 introduced Segregated Witness, which activated on Bitcoin in August 2017.
Bitcoin Improvement Proposal 141 specified Segregated Witness, commonly called SegWit. The change separated transaction signature data, known as witness data, from the transaction’s main data structure. This helped address transaction malleability and allowed more transaction data to fit within Bitcoin’s block-weight rules.
SegWit activated on 24 August 2017 at block 481,824. It also provided an important foundation for later technologies, including the Lightning Network. BIP 141 is sometimes confused with BIP 148, a user-activated soft-fork proposal associated with the same period, or with BIP 91, a separate coordination mechanism. SegWit was a soft fork, meaning upgraded nodes could enforce the new rules while remaining compatible with older versions under the defined conditions.