Time based One Time Password.
Time based standard algorithm, that is used by many token hardware vendors and many smartphone Apps.
The TOTP algorithm uses the “unix time” (seconds since January 1st, 1970) devided by 30 or 60 as a counter and a secret key (seed), to calculate one time passwords. The secret key which is known by the token and the server backend needs to be kept secret.
The TOTP algorithm is define in RFC 6238.