Crypto glossary
Validator attestation
A validator attestation is the signed vote each Ethereum validator casts once per epoch on the current chain head and on the checkpoints used for finality.
On Ethereum's proof-of-stake chain, every active validator is assigned to one slot per epoch, which lasts 32 slots of 12 seconds. In that slot it publishes an attestation: a BLS-signed message that names the block it sees as the head of the chain, plus a source and a target checkpoint. Attestations from the same committee are aggregated so the network can process hundreds of thousands of votes per epoch.
The head vote feeds the LMD-GHOST fork-choice rule, while the checkpoint votes drive Casper FFG finality. Attestations also make up most of a validator's protocol rewards, which is why staking dashboards track effectiveness: whether votes were correct and included quickly. Missed or late attestations cost small penalties and are the most common sign of a validator with connectivity or client problems.
An attestation is not the same as proposing a block; proposals are rare and random, attesting is routine. Signing two conflicting attestations for the same target, or votes that surround one another, is a slashable offence, usually caused by running the same keys on two machines rather than by deliberate attack.
Example
A home staker sees on a beacon chain explorer that its validator missed three attestations overnight during an internet outage and lost a small slice of rewards, with no slashing.