Bitcoin glossary
Requiring multiple keys to spend — no single point of failure or theft.
Multisignature (multisig) locks bitcoin behind multiple keys — for example 2-of-3, where any two of three keys must sign to spend. Losing one key no longer loses the funds, and stealing one key no longer steals them.
It is how serious holders and institutions secure bitcoin: keys on separate devices in separate places, sometimes with a company holding one key for recovery help (collaborative custody) without being able to move funds alone.