OAuth

OAuth is an open standard for authorization, commonly used as a way for Internet users to log in to third party websites using their Google, Facebook, Microsoft, Twitter, One Network, etc. accounts without exposing their password - wikipedia

Generally, OAuth provides to clients a "secure delegated access" to server resources on behalf of a resource owner. It specifies a process for resource owners to authorize third-party access to their server resources without sharing their credentials. Designed specifically to work with Hypertext Transfer Protocol (HTTP), OAuth essentially allows access tokens to be issued to third-party clients by an authorization server, with the approval of the resource owner. The third party then uses the access token to access the protected resources hosted by the resource server.

OAuth is a service that is complementary to and distinct from OpenID. OAuth is also distinct from OATH, which is a reference architecture for authentication, not a standard for authorization. However, OAuth is directly related to OpenID Connect (OIDC) since OIDC is an authentication layer built on top of OAuth 2.0. OAuth is also distinct from XACML, which is authorization policy standard. OAuth can be used in conjunction with XACML where OAuth is used for ownership consent and access delegation whereas XACML is used to define the authorization policies (e.g. managers can view documents in their region).