Multi-factor authentication (also MFA, two-factor authentication, two-step verification, TFA, T-FA or 2FA) is an approach to authentication which requires the presentation of two or more of the three authentication factors: a knowledge factor ("something only the user knows"), a possession factor ("something only the user has"), and an inherence factor ("something only the user knows"). After presentation, each factor must be validated by the other party for authentication to occur.
The RSA SecureID uses two-factor authentication to determine you are who you say you are.