The secret value that authorises spending from an address; possession of it is indistinguishable from ownership.
Why it matters
Every other control in self-custody exists to keep this one value from being read, copied or used without the owner’s intent. A key is not stored on a blockchain and cannot be reset, revoked or reissued by anyone — there is no authority above it. Most losses recorded in the Incident Tracker are failures to keep a key, or its use, under control rather than failures of the cryptography itself.
What you can check
A key can be exposed at generation, at storage, or at use, and those are three different problems. Ask which one a given product actually addresses: a hardware device protects storage and use; it does not protect a key that was weak the moment it was created.
Where it showed up
Records in the Incident Tracker that turn on this: Wintermute vanity address key compromise and Harmony Horizon bridge compromise.
Not advice. Definitions are for understanding, not instruction. Nothing here is financial, legal, tax or security advice.