REST is a set of architectural constraints, not a protocol or a standard. API developers can implement REST in a variety of ways.

In order for an API to be considered RESTful, it has to conform to these criteria:

• A client-server architecture made up of clients, servers, and resources, with requests managed through HTTP.
  
• Stateless client-server communication, meaning no client information is stored between get requests and each request is separate and unconnected.
  
• …
  

from redhat.com/en/rest-api.

In the context of REST APIs, when making multiple identical requests has the same effect as making a single request – then that REST API is called idempotent.

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from restfulapi.net.

• simple structures
  
• straightforward caching possible