Passkey
Also known as: webauthn, fido2
A phishing-resistant replacement for passwords built on public-key cryptography.
A passkey is a keypair stored on your device (or synced across your devices via iCloud Keychain, Google Password Manager, 1Password). Instead of typing a password, you sign a challenge from the site with the private key. There's nothing to phish, nothing to reuse and nothing for the site to leak in a breach.
Passkeys are backed by the FIDO2 / WebAuthn standards and supported natively by every major browser and OS. They're the most impactful account-security upgrade most users can make.