What is SegWit?
Segregated Witness: a 2017 upgrade that moved signatures into a separate part of each payment, making room for more payments.
It switched on in August 2017, at block 481,824. Addresses starting bc1q are SegWit addresses, and payments from them cost less in fees.
It also fixed transaction malleability, the way a payment's ID could be changed before it confirmed, which helped later layers like the Lightning NetworkLightning NetworkA network built on Bitcoin for payments through channels, without recording every payment as its own blockchain transaction..
References
- Bitcoin Improvement Proposals, “BIP-141: Segregated Witness (Consensus layer)”.