The group of keys authorised to attest to events for a chain or a bridge, together with the number of them required to agree.
Why it matters
For a bridge, the validator set is the security model: whoever controls a threshold of it can authorise releases. A set that is small, or whose members share infrastructure, concentrates that power however decentralised the surrounding system appears.
What you can check
Set membership and threshold are normally readable on-chain. What the chain cannot show is whether the keys are genuinely independent — separate operators, separate hosting, separate custody — which is where a nominally distributed set can collapse to a single point.
Where it showed up
Records in the Incident Tracker that turn on this: Ronin bridge validator key compromise and Harmony Horizon bridge compromise.
Not advice. Definitions are for understanding, not instruction. Nothing here is financial, legal, tax or security advice.