Glossary
Archive Node
A full node that retains every historical state, not just the recent ones — letting it answer "what was X's balance at block N?" for any N.
A regular full node prunes old state to keep disk usage manageable — it can validate new blocks against the current state but cannot answer historical state queries.
An archive node keeps every intermediate state forever. On Ethereum mainnet, that's terabytes and growing. Block explorers, analytics providers, and any indexer that needs historical balances or contract storage runs on archive nodes (or relies on a provider that does).