Multi-factor Authentication (MFA) 

is an authentication method that 

requires the user to provide two or more verification factors 

to gain access to a resource 

such as an application, online account, or a VPN

Multi-factor authentication can include two or three different factors, 

which two-factor authentication is always limited to two factors.

Requiring users to authenticate with three factors is more secure,

but users expect an MFA solution to be easy to use.