View key
A Monero key that lets its holder see incoming payments to a wallet without being able to spend them.
Every Monero wallet has two key pairs: a spend key, which authorizes payments, and a view key, which lets a wallet find incoming transactions on the blockchain. Because Monero payouts are private, you need the view key to see them.
The private view key can be shared on purpose. An auditor, accountant or tax adviser can import it into a view-only wallet and see funds arriving without being able to spend anything. Some light wallets also use it on a remote server to scan the chain for you, which trades convenience for some privacy.
A view key alone does not reliably show outgoing payments, and sharing it permanently reveals your incoming history to whoever holds it. To prove a single payment, such as a swap deposit, share the transaction ID and transaction key instead of the view key.
Zcash