Segregated Witness (SegWit) is a soft-fork upgrade to Bitcoin which was activated in 2017. SegWit fixed the problem of transaction malleability, wherein a transaction could have several possible txids.

This upgrade paved the way for the implementation of the Lightning Network, and will open the door for several future upgrades, including Taproot. SegWit also allowed more transactions to be included in a single block, which eased fee pressure and provided a partial scaling solution.