Segregated Witness is a soft fork happened on August 24th, 2017 on the main Bitcoin network. It consists in separating from the transaction information all of the “witness” content, which means the public key and the signature of the party originating the transaction.
The “witness” or more technically the “scriptSig” takes up 60% of the overall space of a transaction. By sending it separately, on a separate channel, a Bitcoin block can now hold more than twice as many transactions. It is also used by Litecoin.