Read get google custom search API key for more information.
The Custom Search JSON API lets you develop websites and applications to retrieve and display search results from Google Custom Search programmatically. With this API, you can use RESTful requests to get either web search or image search results in JSON format.
Custom Search JSON API requires the use of an API key. An API key is a way to identify your client with Google.
cxparameter used by the API.
key=yourAPIKeyto all request URLs. The API key is safe for embedding in URLs, it doesn’t need any encoding.