Segregated Witness, also called SegWit, is a scaling solution for Bitcoin that was first proposed by Dr. Peter Wiulle which aims to increase transaction speed on the Bitcoin network.
Because every transaction on the blockchain needs confirmation of sender funds, known as the signature, transaction speed has slowed with increased adoption of Bitcoin.
The signature takes up blockchain space, and it’s estimated that 65% of the data on the blockchain is comprised of these signatures.
New blocks are created roughly every 10 minutes on the Bitcoin blockchain, so it was proposed that transaction signatures could be segregated from other transaction data, thus freeing up space on the blockchain and increasing transaction speed. SegWit has been very controversial due to differing views on how to scale Bitcoin, and eventually led to the hard fork which created Bitcoin Cash.