WS-Federation Passive Requestor Profile is a Web Services specification - intended to work with the WS-Federation specification - which defines how identity, authentication and authorization mechanisms work across trust realms.
In this context, the web-browser is known as a "passive requestor."
By way of contrast, WS-Federation Active Requestor Profile deals with "active requestors" such as SOAP-enabled applications.
WS-Federation Passive Requestor Profile was created by IBM, BEA Systems, Microsoft, VeriSign, and RSA Security.
[1] This computer networking article is a stub.