"Segregated Witness (SegWit), defined in BIP 141, aims at solving the issue of Transaction Malleability by removing transaction signatures from the transaction hash, meaning that signatures would not be used to compute transaction IDs.

SegWit also changes the way to compute the maximum block size, by defining Block weight that is equal to Base size * 3 + Total size, where Base size is the block size in bytes with the original transaction serialization without any witness related data, while Total size is the block size in byte with transactions serialized including base data and witness data. This can theoretically increase the maximum block size to a bit less than 4MB, even though effective increase is estimated to be closer to 1.7MB.