SegWit (Segregated Witness)

A Bitcoin soft fork activated in August 2017 (BIP 141) that separates transaction signature data (witness) from the transaction body, fixing transaction malleability and increasing effective block capacity from 1 MB to approximately 4 MB via a weight unit system. SegWit introduced a versioned witness program structure that enabled future upgrades like Taproot without additional soft forks. It was a prerequisite for the Lightning Network's secure operation.