Is JSON a language, if not how would it be classified?

JSON is a data format. It could be classified as a language, but not a programming language.

Its relationship to JavaScript is that it shares its syntax (more or less) with a subset of JavaScript literals. The JSON specification defines it completely; it doesn't depend on references to the JavaScript specification.

It's a data interchange format.

JSON (JavaScript Object Notation) is a lightweight data-interchange format.

http://json.org

JavaScript Object Notation is a text-based open standard designed for human-readable data interchange.

Type of format: Data interchange

http://en.wikipedia.org/wiki/Json

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is it only defined in context of a language

Certainly not. It is entirely possible to write applications which use JSON but not JavaScript.