Definition
An AuthnRequest is the SAML 2.0 request message a Service Provider sends to an Identity Provider to initiate authentication — typically via HTTP-Redirect or HTTP-POST binding.
In more depth
SP-initiated SSO starts with an AuthnRequest. It declares the SP's identifier (issuer), preferred NameID format, required AuthnContextClassRef (MFA level), and contains an ID the SP will validate against the InResponseTo field of the resulting assertion.
Not sending an AuthnRequest is what makes IdP-initiated SSO weaker (no InResponseTo binding, replay risks). Modern SAML deployments prefer SP-initiated flows.
Want the work, not just the definition?