Glossary
Slashing
The punishment a proof-of-stake protocol applies when a validator double-signs or otherwise breaks the rules — a portion of their staked tokens is destroyed.
Slashing is the stick in proof-of-stake. If a validator signs two conflicting blocks at the same height, or otherwise violates a provably checkable rule, the protocol burns part of their stake and ejects them.
The threat of slashing is what makes PoS expensive to attack: an attacker needs not only a majority stake but a willingness to see it destroyed. Validator operators correspondingly invest a lot in infrastructure that prevents accidental double-signing during failovers and upgrades.