OpenID Federation 1.0 -
A federation can be expressed as an agreement between parties that trust each other. In bilateral federations, direct trust can be established between two organizations belonging to the same federation. In a multilateral federation, bilateral agreements might not be practical, in which case, trust can be mediated by a third party. That is the model used in this specification. - https://openid.net/specs/openid-federation-1_0.html