What Is a “Witness” in the Context of SegWit Transactions?

The "witness" is a field introduced by the Segregated Witness (SegWit) upgrade that stores the transaction's signature and public key data. SegWit separates this data from the main transaction data, reducing the transaction's size in the block and addressing transaction malleability.

The witness data is still required for validation but is stored in a separate structure, which effectively increases the block capacity.