Private Key
The secret number that authorizes spending from an address; whoever knows it controls the assets, and it cannot be reset.
A private key is a 256-bit random number on the common curves, and wallets rarely store just one: they derive a tree of keys from a single seed using hierarchical deterministic derivation, so one backup covers every address. Signing proves possession of the key without revealing it, and the network checks the signature rather than the identity of the signer. Because there is no issuer, loss is permanent and theft is irreversible, which is why hardware wallets keep the key inside a device that signs internally and never exports it. Multisignature arrangements and threshold signature schemes split control across several keys or several parties so no single compromise is enough, and these are the standard patterns for institutional and shared custody.
In practice
A hardware wallet displays the transaction details on its own screen and signs internally, so a compromised computer can request a signature but cannot extract the key.
The common misunderstanding
A private key is not a password that a service can reset; nobody else holds a copy, so no support channel can restore or revoke it.