Transaction key

The Monero transaction key (often called the tx_key, tx key, or transaction secret key) is used, in conjunction with other information, to prove a Monero payment.

The Monero transaction key is not stored anywhere on the blockchain. It is only generated and stored locally from the same wallet used to create the transaction. Thus, if you lose that wallet, you will lose the transaction key as well, even if you still have the mnemonic seed.