Light Client
Software that checks a blockchain using only block headers and small proofs instead of downloading the whole chain.
A light client keeps the chain of headers, which is small, and asks peers for a Merkle proof when it needs to confirm that a specific transaction was included in a specific block. This makes verification possible on a phone or inside a browser extension, at the cost of extra assumptions: the client trusts that it is seeing the real header chain and cannot check that the blocks themselves obey every rule. Ethereum supports light clients through a rotating sync committee whose signatures a small client can follow, and trust-minimized bridges are built from light clients running inside contracts on another chain. Readers meet them inside mobile wallets and cross-chain messaging systems.
In practice
Bitcoin's simplified payment verification lets a wallet confirm that a payment was included in a block by checking a Merkle proof against a header it already holds.
The common misunderstanding
A light client is not just a wallet with fewer features; it verifies less than a full node, so it depends on assumptions a full node does not need.