SegWit

Segregated Witness, activated in August 2017, separates the signature data, the “witness”, from the part of a transaction that determines its ID. Before SegWit, a third party could alter a signature’s encoding and change a transaction’s ID without invalidating it. That malleability made chains of unconfirmed transactions, which Lightning depends on, unsafe.

SegWit also replaced the 1 MB block size limit with a 4 million weight-unit limit that discounts witness data, raising effective capacity. It introduced versioned witness programs, the upgrade path later used by Taproot. Addresses starting with bc1q are SegWit addresses.