Archive node

An archive node is a blockchain node that retains the complete historical state of the chain, allowing queries about account balances and contract storage at any past block height.

A full node keeps recent state and can validate the chain, but pruning discards old state. An archive node keeps every historical state, which is what makes 'balance at block N' or trace/debug queries possible. Archive nodes are expensive to run, which is why archive RPC is often a premium capability — serving it from your own archive nodes removes the reseller markup.