SPV
Also: Simplified Payment Verification, light client
network · intermediate
Verifying transactions without running a full node — by downloading only block headers and merkle proofs.
SPV was sketched in Satoshi's whitepaper, Section 8. The idea: a light client downloads the chain of 80-byte block headers (currently ~70 MB total, vs ~600+ GB for the full chain) and asks peers for merkle proofs that a given transaction is included in a given block.
SPV gets you most of what a node gets — proof that a transaction is in the chain, and an estimate of how many confirmations it has. What it doesn't get you: validity of *other* transactions. An SPV client trusts that peers are honest about which blocks exist; it can't independently verify the supply hasn't been inflated. A fooled SPV client could be paid in fake bitcoin.
The pragmatic compromise is BIP-157/158 ("compact block filters"): the node serves a small filter per block; the client uses it to download only blocks containing transactions relevant to its wallet. Privacy-preserving, low-bandwidth, but the trust model is still weaker than a full node.