Extensible Data Notation. A data format describing how to serialize and deserialize a set of scalars (numbers, rationals, decimals, big integers, instants/date-times, booleans, nil, strings, keywords, symbols) and collections (vectors, sets, maps, lists), with support for custom types. Cross-language parsers exist (Ruby, Java, etc.). Clojure uses EDN as the syntax for its own source code, compiled via AST to bytecode — not because EDN is Clojure but because Clojure chose EDN. Described in the talk as 'safe YAML' (typed extensibility) or 'JSON++' (supports vectors, sets, lists, instances that JSON can't represent). Used ubiquitously for inter-service communication, stored procedures, configuration.