Authentication Factor
One of five categories of evidence — something you know, have, are, do, or your context — that backs a claim of identity; combining independent categories is what makes MFA meaningfully stronger than a single factor.
Stacking two passwords isn’t MFA — they’re both “something you know,” so a single leaked database compromises both at once. Real multi-factor strength comes from combining categories that fail independently: a phished password doesn’t also hand over a hardware key sitting on someone’s desk.