Glossary
Event / Log
A structured, indexed record a contract emits into a block — cheap to write, expensive to read on-chain, and the primary feed for off-chain indexers.
When a contract calls emit, the EVM records a log entry into
the block: up to four indexed topics (queryable by node filters)
plus an arbitrary data payload. Logs are never readable by other
contracts — they are write-only from the chain's point of view.
Off-chain, logs are how indexers, subgraphs, and explorers
reconstruct application state in real time. ERC-20 Transfer and
ERC-721 Transfer events are the most heavily indexed signals
on Ethereum.