One Time Password.
This is a password that can be used only once. It is generated by an external device and used to assure a secure authentication with two factors.
If the user enters the correct one time password and this password can only be generated by the one token, the system “knows” that the user must be in possession of this token. Thus the user can only login with a password he knows and the token he possesses. This is a way of two factor authentication.