A validator is a node running in a proof-of-stake blockchain. Its job is to propose new blocks when selected and to attest to the validity of blocks proposed by other validators. Together, the validator set reaches consensus on the chain's state.
Becoming a validator requires: - Running the node software 24/7 with low downtime. - Staking a minimum amount of the native token (32 ETH on Ethereum, 1 SOL on Solana, any amount on Cosmos). - Maintaining a secure key-management setup — validator keys must sign blocks but must not be exposed to the internet without hardware protection.
Validators earn rewards denominated in the native token, roughly proportional to their stake weight. They lose stake via **slashing** if they misbehave: proposing conflicting blocks, going offline for extended periods, or signing equivocating attestations.
The number of active validators is typically capped or bottlenecked by hardware requirements. Ethereum has ~1 million active validators; Solana has ~2,000. Bigger sets are more decentralized in theory but add consensus overhead.