Crypto glossary
Light client
A light client is node software that checks block headers and cryptographic proofs instead of downloading and re-executing the whole blockchain.
A full node verifies every transaction since the genesis block, which needs significant storage and bandwidth. A light client keeps only the chain of block headers, or a compact summary of them, and asks full nodes for proofs that a specific transaction or account balance is included. It can then verify those proofs mathematically against the headers it trusts.
Light clients make blockchain verification possible on phones, browsers and small devices. Bitcoin wallets have used the simplified payment verification idea since the early years, and Ethereum's proof of stake added sync committees, rotating groups of validators whose signatures let light clients follow the chain cheaply. Trust-minimised bridges also embed light clients of other chains inside smart contracts.
The savings come with weaker guarantees. A light client usually cannot detect an invalid transaction hidden inside a block that has enough work or signatures behind it, and it depends on honest full nodes being reachable. Many wallets marketed as lightweight simply query a company's server and do no verification at all.
Example
A browser wallet running an Ethereum light client verifies that a token balance returned by an RPC provider matches the state root signed by the sync committee, instead of trusting the provider blindly.