State Pruning

State pruning is a node discarding historical data it no longer needs to validate what comes next. A chain's current state — every balance and every contract's storage — is what a node must know to check the next block; the record of every intermediate state along the way is optional. Pruning throws that away and keeps the node's disk use bounded, which is the difference between a full node that fits on an ordinary machine and one that does not. Nothing about security is lost: a pruning node still verified every block it processed, and it will still reject an invalid one. What is lost is the ability to answer questions about the past, which is why an application needing historical queries has to talk to an archive node instead. Whether a chain can be validated on affordable hardware is one of the more practical measures of how decentralised it is in practice.