zetta- (symbol Z) is an SI prefix in the SI (system of units) denoting 1021 or 1 000 000 000 000 000 000 000. Adopted in 1991, it comes from the Latin septem, meaning seven, because it's equal to 10007. A prefix of the same value, hepa-, was informally introduced a few years before the promulgation of zetta-. It was formed from the Greek ἑπτά, (hepta), also meaning seven. It never received official sanction and is now obsolete.… (More on Zetta-)