Node

Also called: full node, Bitcoin node, bitcoind, pruned node, Bitcoin Core node.

A Bitcoin node is a computer running Bitcoin software that downloads, checks and relays blocks and transactions; a full node verifies every rule itself.

A node is any computer running software that speaks the Bitcoin peer-to-peer protocol. A full node downloads every block and checks it against all the consensus rules on its own, so it never has to trust a miner, an exchange or another node about what is valid. Bitcoin Core is the most widely used implementation.

Types of node

Archival full node: keeps the whole blockchain and serves old blocks to peers (service bit NODE_NETWORK).
Pruned node: validates everything but deletes old block data. BIP 159 lets it advertise NODE_NETWORK_LIMITED and serve at least the last 288 blocks.
Light client: checks only block headers and relies on full nodes for the rest, using SPV with Bloom filters (BIP 37) or compact block filters (BIP 157, BIP 158).

What nodes do

Besides validating, nodes keep a mempool of unconfirmed transactions and relay transactions and new blocks to peers. Many BIPs define this network layer: compact block relay (BIP 152), Tor v3 and I2P addresses (BIP 155), wtxid-based relay (BIP 339) and encrypted connections (BIP 324, see v2 transport). The network topic lists them all.

Consensus vs policy

A node applies two kinds of rules. Consensus rules decide whether a block is valid and must be identical everywhere. Policy rules decide which unconfirmed transactions a node accepts and forwards, and can differ from node to node. That is why a transaction can be relayed by some nodes and refused by others, yet still be valid once it is mined.

Running your own node is how you check incoming payments yourself instead of asking someone else's server.

BIPs that define Node

BIP 159: NODE_NETWORK_LIMITED service bit: Deployed. Defines a signal that pruned nodes, which keep only recent blocks to save disk space, can use to advertise that they can serve recent history. This lets pruned nodes contribute more to the network…
BIP 324: Version 2 P2P Encrypted Transport Protocol: Deployed. Introduces a new, encrypted way for Bitcoin nodes to talk to each other, so internet providers and passive observers cannot read or easily tamper with the traffic. It also slightly reduces bandwidth…
BIP 152: Compact Block Relay: Deployed. Introduces compact blocks, where nodes announce a new block using short transaction identifiers instead of sending every transaction again. Since peers usually already have most transactions, this…
BIP 155: addrv2 message: Deployed. Adds a new message for sharing node addresses that are longer than the old format allows. This was necessary to support newer Tor onion addresses, I2P, and other privacy networks, helping nodes…
BIP 157: Client Side Block Filtering: Deployed. Defines a protocol for lightweight wallets in which full nodes publish compact summaries (filters) of each block, and wallets download them to check locally which blocks matter. Unlike the older…
BIP 37: Connection Bloom filtering: Deployed. Lets lightweight wallets hand a full node a Bloom filter (a compact, approximate summary of what they care about) so they receive only relevant transactions. This made phone wallets practical,…
BIP 339: WTXID-based transaction relay: Deployed. Nodes share new transactions by announcing short identifiers, and this change lets them use the "wtxid", an identifier that also covers a transaction's SegWit signature data. The older identifier…

Related terms

Consensus rules: Consensus rules are the checks every full node applies to decide whether a block or transaction is valid; a block that breaks one is rejected.
Mempool: The mempool is a node's waiting area for valid, unconfirmed transactions; miners choose from it, usually by fee rate, when they build a block.
SPV (Simplified Payment Verification): SPV (Simplified Payment Verification) is how light wallets check payments using only block headers and Merkle proofs, without downloading full blocks.
v2 P2P transport: v2 P2P transport (BIP 324) encrypts connections between Bitcoin nodes, so eavesdroppers can't read the traffic or easily recognize it as Bitcoin.