node
A node is a computer that participates in the Bitcoin network by storing a copy of the blockchain and verifying every transaction and block against Bitcoin's rules. Nodes are the backbone of Bitcoin's decentralization.
A Bitcoin node is a computer running the Bitcoin software that independently downloads, stores, and validates the entire history of the blockchain. When a new transaction is broadcast to the network, nodes check whether it follows Bitcoin's rules: does the sender have sufficient funds, is the signature valid, is the transaction formatted correctly? Only transactions that pass these checks are relayed further. When a new block is found by a miner, nodes verify it the same way before accepting it into their copy of the blockchain.
The critical word here is independently. A node does not trust miners, exchanges, or other nodes. It verifies everything itself using the rules encoded in the Bitcoin software. This is why running your own node is considered the highest form of Bitcoin sovereignty. When you use a node you control, you are not relying on anyone else's version of the truth. You know your balance is correct because you checked it yourself against the full transaction history.
Nodes also protect the network from rule changes. If a group of miners or developers tried to change Bitcoin's core rules, such as increasing the 21 million supply cap, nodes running the original software would reject those blocks. The economic majority of nodes defines which chain is the real Bitcoin. This is what makes Bitcoin resistant to manipulation: no single actor, no matter how powerful, can change the rules without convincing the majority of node operators to voluntarily update their software.