react-jsonschema-form/docs/definitions.md

Schema definitions and references

This library partially supports inline schema definition dereferencing, which is Barbarian for avoiding to copy and paste commonly used field schemas:

{
  "definitions": {
    "address": {
      "type": "object",
      "properties": {
        "street_address": { "type": "string" },
        "city":           { "type": "string" },
        "state":          { "type": "string" }
      },
      "required": ["street_address", "city", "state"]
    }
  },
  "type": "object",
  "properties": {
    "billing_address": { "$ref": "#/definitions/address" },
    "shipping_address": { "$ref": "#/definitions/address" }
  }
}

(Sample schema courtesy of the Space Telescope Science Institute)

Note that it only supports local definition referencing; we do not plan on fetching foreign schemas over HTTP anytime soon. Basically, you can only reference a definition from the very schema object defining it.