Glossary
Subgraph
An open specification for indexing a slice of on-chain data — schema, event handlers, manifest — deployed to The Graph or an alternative indexer.
A subgraph defines: which contracts on which chain to follow, which events / call handlers to track, and a GraphQL schema for the resulting data. The runtime (an indexer node) processes blocks in order, runs the handlers (written in AssemblyScript), and writes entities into a database that backs GraphQL queries.
Subgraphs were originally a feature of The Graph, but the specification is open and other indexers (Goldsky, Subsquid, self-hosted Graph Node) can run the same artifacts.