Transaction Hash (TXID)
A transaction hash, or TXID, is the unique fingerprint of a transaction used to look it up on the blockchain.
In depth
It is produced by hashing the transaction data, so it is fixed the moment the transaction is created. Paste a TXID into a block explorer and you can see the sender, receiver, amount, fee and confirmation count. Exchanges ask for the TXID when you report a deposit that has not shown up.
Why it matters
The TXID is your receipt — the one piece of information that proves a payment exists.