Validator
In short: A validator is a node responsible for validating transactions and adding new blocks to a blockchain, most often on a proof-of-stake network.
A validator is a node responsible for validating transactions and adding new blocks to a blockchain, most often on a proof-of-stake network. Validators check that transactions follow the rules, propose blocks when selected, and vote on blocks proposed by others.
Why it matters
Validators do the job that miners do on proof-of-work chains, but they secure the network with staked tokens instead of computing power. Running one usually means staking the required amount of the native token and keeping a server online around the clock with up-to-date software. Reliable validators earn rewards, while those that break the rules, such as by signing two conflicting blocks, can be slashed and lose part of their stake. The number and independence of validators is one way to judge how decentralized a network is.
Example
On a proof-of-stake chain, the protocol selects a validator, weighted by stake, to propose the next block. The validator bundles pending transactions, checks each signature and balance, and broadcasts the block. Other validators re-check it and sign their approval. Once enough of the total stake has approved, the block is final, and the validators involved receive their share of the rewards.