SegWit, or Segregated Witness, refers to an update that changed the transaction format in the Bitcoin protocol. Its purpose as a protocol upgrade was to protect against transaction malleability and increase block size capacity, which would enable more transactions to fit inside a block. Transaction malleability describes the risk that minor elements of transaction information can be altered, potentially invalidating new blocks. The proposal of SegWit came from Peter Wuille in 2015, and it was successfully activated in Bitcoin Core by 2017.