Definition
FIDO2 is a set of open authentication standards (WebAuthn + CTAP2) that enable phishing-resistant passwordless authentication using public-key cryptography.
FIDO2 combines two specifications: the Web Authentication API (WebAuthn, a W3C standard exposed in browsers) and the Client-to-Authenticator Protocol (CTAP2, governing how authenticators communicate with the platform). Together they enable a registration ceremony (creating a credential bound to the relying party + the authenticator) and an authentication ceremony (proving possession via a signed challenge).
FIDO2 is the technical foundation under "passkeys" — discoverable, syncable credentials that work across devices. Because authentication is bound to the relying party's origin, FIDO2 credentials cannot be phished.