A verifiable statement from an IdP to an RP that contains information about an end user. Assertions may also contain information about the end user’s authentication event at the IdP.
Source(s):
FIPS 201-3
under Assertion
A statement from a verifier to an RP that contains information about a subscriber. Assertions may also contain verified attributes.
Source(s):
NIST SP 800-63-3
under Assertion