Resource owner (the User):
An entity capable of granting access to a protected resource (for example end-user).

Resource server (the API server):
The server hosting the protected resources, capable of accepting responding to protected resource requests using access tokens.

Client:
An application making protected resource requests on behalf of the resource owner and with its authorization.

Authorization server:
The server issuing access tokens to the client after successfully authenticating the resource owner and obtaining authorization.