Glossary
WETH
Wrapped Ether — an ERC-20 representation of native ETH, used wherever a contract expects a token-standard interface that native ETH doesn't satisfy.
ETH predates ERC-20 and doesn't implement the standard. Most DEXes, lending markets, and DeFi primitives are written against ERC-20. WETH closes the gap: deposit ETH into the WETH contract, receive WETH 1:1; burn WETH, get ETH back.
The WETH9 contract on Ethereum mainnet — deployed in 2017 — holds a large fraction of all ETH in circulation at any given time. Many other chains have their own WETH-equivalent (WAVAX, WMATIC, WSOL) following the same pattern.