Transaction ID (txid)

A transaction ID (txid) is the unique identifier of a bitcoin transaction, computed by hashing the transaction's data twice with SHA-256 and displayed as a 64-character hexadecimal string. Pasting a txid into any block explorer locates the transaction; the famous first transfer, Satoshi Nakamoto's 10 BTC to Hal Finney in block 170, has a txid beginning f4184fc5.

How it works

Because the txid is a hash of the transaction's contents, any change to those contents produces a different txid. That property once caused real trouble: signatures could be altered by third parties in cosmetically different but still valid ways before confirmation, changing the txid without changing the payment, a flaw called transaction malleability. Malleability complicated any protocol that chained unconfirmed transactions together, including early payment channels, and featured in disputes around the Mt. Gox collapse. The 2017 SegWit upgrade fixed it by moving signature data outside the portion of the transaction that the txid commits to, which is part of why the Lightning Network became practical only after SegWit activated.

Why it matters

The txid is bitcoin's native receipt. Anyone can prove a payment occurred by citing it, auditors can trace fund flows through chains of txids, and no institution's cooperation is needed to verify any of it. Physical gold offers no equivalent, as a bar's provenance lives in fallible paperwork; every bitcoin that moves writes its own permanent, globally checkable record.