SAML 2.0 and Liberty WSF 2.0 apply this term to the SAML protocol endpoint at an !IdP that a client can contact to obtain a SAML authentication assertion for use at an SP. The client delivers it an AuthnRequest message (with a browser client, this is from an SP, not created by the client) and authenticates somehow so that the !IdP knows the identity at the client. If the conditions are acceptable, then the !IdP will issue an assertion for use by the client to authenticate itself to the SP.
Shibboleth has adopted this terminology for its own use of SAML 1.1 to describe the location in the !IdP software that the browser is sent with its redirect-based AuthnRequest to initiate the BrowserPOST or BrowserArtifact profiles.
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