Glossary
SegWit
"Segregated Witness" — a 2017 Bitcoin soft fork that moved signature data out of the transaction body, expanding effective block size and fixing malleability.
Before SegWit, Bitcoin transactions stored signature data ("witnesses") alongside the inputs they unlocked. SegWit separated witnesses into their own structure, fixed a long-standing transaction-malleability bug, and effectively expanded the per-block data limit by re-weighting how block-size accounting works.
The soft fork activated in August 2017 after a contentious multi-year governance battle that nearly split the chain. It paved the way for the Lightning Network and for the later Taproot upgrade.