Light Client

A light client is blockchain software that verifies transactions without downloading the entire chain, storing only compact data such as block headers and requesting proofs for the specific transactions it cares about. This lets phones, browsers, and low-powered devices interact with a blockchain that would otherwise require hundreds of gigabytes of storage and days of syncing.

The classic design, described in the original Bitcoin whitepaper as simplified payment verification (SPV), works by downloading the header of each block and then asking full nodes for a Merkle proof that a given transaction is included in one of those blocks. The light client can check the proof mathematically, so full nodes cannot fabricate transactions — though a light client does trust that the chain of headers with the most accumulated work or stake follows the network's rules. For example, a mobile Bitcoin wallet using SPV can confirm that a payment to you was mined without storing the full ledger.

A common misconception is that light clients are just wallets that query a company's server; a true light client verifies proofs itself, whereas many popular wallets simply trust a provider's API, which is weaker. Light clients trade some security assumptions for accessibility: they cannot detect every kind of invalid block the way a full node can, but they dramatically lower the cost of using a blockchain somewhat trustlessly, and newer designs using zero-knowledge proofs aim to shrink that trust gap further.