TXID

A Transaction ID or TXID is a double SHA256 hash of the serialised transaction. TXIDs are not part of the the transaction, as the hash cannot be generated until the transaction is complete. A TXID and VOUT index are used to reference UTXO when they are added to a transaction as an input.

The double SHA256 hash can be used to enable users to know that a corresponding party has a copy of the full TX by requesting the intermediate hash, which can only be generated by hashing the full transaction. This 'proof of possession' is present across Bitcoin and enables complex functionality and checking of resources when Bitcoin is being used in a fully peer to peer environment.