A blockchain validator is a participant with a protocol role in checking and voting on proposed blockchain activity. The details vary by network. In this article, the example is Ethereum proof of stake.
What an Ethereum validator does
Ethereum’s proof-of-stake documentation describes validators as checking that proposed blocks follow the rules and that their contents and signatures are valid. Validators issue attestations, which are votes about the chain, and a validator selected for the relevant duty may propose a block.
These duties are part of Ethereum’s consensus mechanism. “Validator” is therefore not a general synonym for every node, block producer or person who reads blockchain data.
Why the network name matters
Different blockchains can use different consensus mechanisms and assign different responsibilities to participants. An explanation should identify the protocol rather than assume that an Ethereum validator’s role applies everywhere.
What this article does not cover
This definition does not explain how to join a network, configure software, make a deposit, choose a provider or assess returns. Those are separate practical or financial topics, not part of the core terminology.
In short
On Ethereum, a proof-of-stake validator checks proposed blocks, issues attestations and may propose a block when selected for that duty.