The id produced on the blockchain that represents a particular transaction. Imagine Amarantha sends Bartholomew some ETH. There will be a sending address, a receiving address and a transaction hash to represent the transaction. Amarantha may share the transaction hash with Bartholomew as proof the transaction or payment was made. Bartholomew could look up the transaction hash on the blockchain via a block explorer and verify the transaction was made. Or he could ask his brother Bob or Amarantha’s sister Alice to do it too, of course.
“If you think you made some bad crypto trades, this is the actual hash of the transaction to send 10,000 bitcoin (around $500 million in 2022) for pizza in 2010 – https://www.blockchain.com/btc/tx/a1075db55d416d3ca199f55b6084e2115b9345e16c5cf302fc80e9d5fbf5d48d”« Dictionary Menu