Multi-factor authentication (MFA) enhances security by requiring two or more verification methods to validate a user's identity before access to resources is granted. This could include something you know (password), something you have (a trusted device that is not easily duplicated, like a phone), or something you are (biometrics). Single sign-on (SSO) simplifies the sign-in process by allowing users to access multiple applications with one set of credentials but does not inherently provide multiple forms of verification. Passwordless methods replace traditional passwords with another form of verification and may not alone constitute multiple factors unless combined with another method.