Multi-factor authentication
Also known as MFA, 2FA
Requiring two or more different kinds of proof to sign in, so a stolen password alone is not enough.
The factors are something you know, something you have, and something you are. Combining categories is what matters: a password plus a security question is still one category and adds almost nothing.
MFA blocks the overwhelming majority of account compromise attempts, which is why it is the single highest-value control for most businesses. Not all implementations are equal, though, and SMS codes and push approvals are both bypassable by determined attackers.
Where this comes up
Multi-factor authentication sits inside our identity and access work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.