Merge pull request #462 from json-schema-org/instance-schema

Add glossary entries for schema and instance.

Author: Julian

learn/glossary.md

@@ -24,6 +24,15 @@ While future drafts may introduce new behavior or changes to existing behavior,
 
 The current list of drafts can be found [here](https://json-schema.org/specification-links.html#published-drafts).
 
+### instance
+
+A piece of JSON data which is to be validated by a [schema](#schema).
+
+JSON Schema can be used to validate JSON values of any type (as well as values from many JSON-like formats which can be reasonably represented as JSON).
+
+The JSON Schema specification makes no broad assumptions about the structure of instances themselves beyond those of the JSON specification itself.
+In particular it does not reserve any properties within a JSON object for its own use, or require parsers of JSON to support features beyond those already mandated of JSON implementations.
+
 ### normative
 
 In the context of JSON Schema, and formal specifications more broadly, a document which outlines standardized behavior.
@@ -36,3 +45,13 @@ This page for instance, not being a normative document, is not able to proscribe
 #### See also
 
 * [normative](https://developer.mozilla.org/en-US/docs/Glossary/Normative) and [non-normative](https://developer.mozilla.org/en-US/docs/Glossary/non-normative) in the Mozilla Glossary, and its links
+
+### schema
+
+A document, written according to the proscribed structure of the JSON Schema specification, which can be used to validate [instances](#instance).
+
+The rules constituting what schemas are *valid* JSON Schemas, as well as the rules governing their behavior when validating instances, are defined by the JSON Schema specification.
+
+Strictly speaking, according to the specification, schemas are themselves JSON documents, though it is somewhat common for them to be authored or maintained in other languages which are easily translated to JSON, such as YAML.
+
+In recent [drafts](#draft) of the specification, a schema is either a JSON object or a JSON boolean value.