1) Unit of information 
The zettabyte is a multiple of the unit byte for digital information. The prefix zetta indicates multiplication by the seventh power of 1000 or 1021 in the International System of Units (SI). Therefore one zettabyte is one sextillion (one long scale trilliard) bytes. The unit symbol is ZB. A related unit, the zebibyte (ZiB), using a binary prefix,...
Unit of storage, details ...
A rather large number of bytes. 10 to the power of 21 to be precise. People who use this term in conversation are likely to be quite insecure.