A master public key in HD wallets used to derive many child public keys/addresses without exposing private keys.
In BIP32 HD wallets, an xpub can derive child public keys for a branch, allowing watch-only wallets and address generation without access to private keys. It must be kept private enough to preserve on-chain privacy.
"An accounting team uses the xpub to monitor incoming payments without being able to spend funds."
All terms and definitions may update as the Cryptionary improves.