Glossary
Light Client
A blockchain client that verifies block headers and Merkle proofs instead of executing every transaction, letting it run on phones and embedded devices.
A full node downloads, stores, and re-executes every transaction. A light client only downloads block headers and asks full nodes for Merkle proofs of the specific data it needs — an account balance, a transaction inclusion, a contract storage slot.
This drops storage from hundreds of gigabytes to a few megabytes and CPU to nearly nothing, in exchange for trusting the full node to relay correct proofs. Mobile wallets and embedded devices are typically light clients.