Zetta is a decimal unit prefix in the metric system denoting a factor of 1021 or 1000000000000000000000. The prefix was added as an SI prefix to the International System of Units (SI) in 1991 and has the symbol Z. Zetta is evocative of the Latin numeral septem, meaning "seven". Therefore it was used to denote the seventh power of 1000. 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 Hepta never received official sanction and is obsolete. Zeta (ζήτα), although being the 6th letter of the Greek alphabet, denotes a value of seven (7) when used as a numeral. Examples: \n* The mass of Earth's atmosphere is approximately 5 zettagrams (Zg). \n* The volume of seawater in the Earth's oceans is approximately 1.369 zettalitres (Zℓ). \n* Avogadro's constant is 602.2 zettamol−1 (Zmol−1). \n* Estimates of the diameter of the Milky Way Galaxy are between 0.9 and 1.7 zettametres (Zm). \n* A zettabyte (ZB) is a quantity of information or information storage capacity equal to 1021 bytes, 10007 bytes, 1,000 exabytes or 1 sextillion bytes. The zebibyte (ZiB) is a related unit that uses a binary prefix, and means 10247 bytes.