In units of measurement, a factor of 1021 or one sextillion.
Based on Latin septi‑, seven, with the last letter changed to match other SI prefixes.
The association with seven is that it’s the seventh unit in the SI (Système International) set of prefixes (see Number words), one thousand to the power of seven.
This unit so large that terms containing it are infrequently encountered. In science writing the numerical equivalent is normally used instead. However, it does have some currency in popular science and science fiction. Examples are zettatonne (the mass of the Earth is about 5.9 zettatonnes) and zettametre (20 zettametres is roughly the distance to the Andromeda galaxy).
Easily the most common usage is in computing, where extremely rapid increases in storage capacity and transmission rates have brought their quantities into the unit’s range. Hence zettabyte (the world’s internet traffic reached one zettabyte in 2016).