Definition format (JSON-Schema-like) describing the fields, types, constraints, and required attributes of records in a given collection (e.g. app.bsky.feed.post requires text and createdAt, with text up to 300 unicode characters / 3000 graphemes). Lexicons are public, so third parties can read any record and interoperate by matching on its lexicon. Records may include undefined extra fields, but different apps must avoid naming collisions. Shared lexicons across projects (e.g. Standard Site for blog posts) enable cross-platform tools such as multi-platform search.