Glossary
Node
A computer running the blockchain client software — validating blocks, relaying transactions, and serving as a peer to other nodes.
A node is any machine running the network's client software. Full nodes verify every block against the protocol rules and keep the full state; archive nodes additionally retain every historical state; light nodes only check headers and rely on full nodes for proofs.
The number, diversity, and geographic spread of independent nodes is one of the cleanest measures of how decentralized a chain actually is in practice.