What is Relying Party (RP)

An application that accepts tokens from an STS is called as a Relying Party (or RP). In modern scenarios, web applications use WIF and accept tokens from an STS to manage authentications process.

These tokens acts a proof that user has been authenticated by our application. Thus, our application relies on an external service i.e. an STS to provide Access Control and thus our application is termed as a Relying Party.

More Explanation about Security Token Service.