Node
A computer running a blockchain's software, independently validating every rule — the physical substance of decentralization.
A node stores the blockchain and verifies every block and transaction against the protocol’s rules, rejecting anything invalid regardless of who sent it. Thousands of independent nodes are what “decentralized” physically means: rules enforced by everyone, changeable by no one unilaterally.
Running a Bitcoin or Ethereum full node takes consumer hardware and ordinary bandwidth — deliberately accessible, since validation cost gates who can independently audit the money. Nodes differ from miners/validators: nodes check the rules; miners and validators order transactions within them. The nodes’ collective veto is why even overwhelming hash power can’t print extra coins.