What Is the Difference between ‘Slashing’ and an ‘Inactivity Leak’ in PoS?

Slashing is an active, immediate penalty (seizure of stake) for explicitly malicious or highly negligent behavior, such as double-signing. An inactivity leak is a passive, gradual penalty (slow reduction of stake) applied to validators who are offline or fail to attest to blocks.

The leak is designed to ensure the network can finalize transactions even with a large portion of validators offline, by gradually reducing their stake until they are a minority.