21 BTCProof of reserves
In plain words
An exchange's attempt to show it holds the customer coins it owes.
See also
- Mt. Gox (2014)The largest exchange of its time stopped withdrawals and filed for bankruptcy; about 850,000 BTC were reported missing after years of undetected theft.
- KYC"Know your customer": identity checks required by regulated exchanges.
- Hot / cold walletHot: keys on an internet-connected device. Cold: keys kept offline.