Segregated Witness (segwit) defines a transaction format introduced in 2017 which doesn't include signatures in the transaction identifier (txid).
Among other benefits, SegWit solves third-party transaction malleability, reduces the computational complexity of signature verification, and introduces Script versioning.
The Segregated Witness Wallet Development Guide provides an overview about working with SegWit.
Segwit defines new output types which are the only outputs that can take advantage of these benefits. This was done in order to maintain backwards compatibility as Segwit is deployed as a soft fork. Segwit was activated on block 481824.