A master private key in HD wallets that can derive all child private keys; extremely sensitive.
In BIP32 HD wallets, the xprv controls an entire branch (or the whole wallet). Anyone who gets it can derive all descendant private keys and spend all associated funds.
"They kept the xprv in cold storage and only exported an xpub to their watch-only wallet for monitoring."
All terms and definitions may update as the Cryptionary improves.