full node
A full node is a computer running the blockchain's software that keeps a complete copy of the ledger and independently checks every rule for itself. Rather than asking someone else whether a transaction or block is valid, a full node verifies it from scratch — confirming each signature, making sure no coins are spent twice, and rejecting anything that breaks the protocol. It is the network's grassroots referee, and there are thousands of them.
This is the muscle behind a blockchain's trustlessness and decentralization. Because each full node enforces the rules on its own, no miner, developer, or company can sneak in an invalid transaction or change the supply of coins — the moment they tried, every full node would simply reject the bad block. The shared truth of the network is, in effect, whatever the majority of independent full nodes agree is valid, which is why having many of them spread around the world keeps power from concentrating.
Full nodes differ from lighter clients that save space and bandwidth by storing only block headers and asking full nodes for the rest; those light clients trade some self-sufficiency for convenience. Running your own full node is the strongest way to use a blockchain, because you rely on no one else's word — your software checks the entire history and tells you the truth directly.
Full nodes verify and relay the rules but do not necessarily produce new blocks — that extra job belongs to miners or validators, who are themselves a special kind of full node.