zetta (symbol Z)'s 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 French prefix septo, meaning seven, because it's equal to 10007. A prefix of the same value, hepta, was informally introduced a few years before the promulgation of zetta. It was formed from the Greek ἑπτά, (hepta), also meaning seven. Hepta never received official sanction and's now obsolete. Footnotes… (More on Zetta-)