Validator
A node that participates in a Proof of Stake network's consensus process by staking tokens to propose and attest to blocks.
In a Proof of Stake network, validators are nodes that have deposited the required stake (32 ETH on Ethereum) to participate in consensus. They are randomly selected to propose new blocks and are required to attest to blocks proposed by others. In return, they earn staking rewards — newly issued tokens plus transaction fees.
Running a validator requires reliable uptime (being offline results in small 'inactivity penalties'), correct software configuration, and the operational security to protect signing keys. Many token holders delegate to staking services (Lido, Rocket Pool, exchanges) rather than run their own validators.
The number of active validators is a rough measure of network decentralisation. Ethereum has over 1 million validator deposits but a more concentrated picture of actual entities running those validators, with Lido and exchanges controlling substantial shares.