Sure, here's an example of an OpenAPI definition in JSON format:

{
  "openapi": "3.0.0",
  "info": {
    "title": "Pet Store API",
    "version": "1.0.0",
    "description": "This is a sample server Petstore server",
    "termsOfService": "http://example.com/terms/"
  },
  "servers": [
    {
      "url": "https://petstore.example.com"
    }
  ],
  "paths": {
    "/pets": {
      "get": {
        "summary": "List all pets",
        "responses": {
          "200": {
            "description": "A list of pets",
            "content": {
              "application/json": {
                "schema": {
                  "type": "array",
                  "items": {
                    "type": "object",
                    "properties": {
                      "id": {
                        "type": "integer",
                        "format": "int64"
                      },
                      "name": {
                        "type": "string"
                      },
                      "tag": {
                        "type": "string"
                      }
                    }
                  }
                }
              }
            }
          }
        }
      }
    }
  }
}

This example defines a simple Pet Store API with a single endpoint to get a list of pets. It includes information about the API version, the server URL, and the endpoint's operation, responses, and data schema.