If we really want to be clear, a Yubikey is a token capable of running applets including, but not limited to FIDO U2F. Other applets commonly used include PIV (

If we really want to be clear, a Yubikey is a token capable of running applets including, but not limited to FIDO U2F. Other applets commonly used include PIV (pkcs11), OpenPGP, OATH, and Yubi's own OTP applet.