yotta- (symbol Y) is an SI prefix in the SI (system of units) denoting 1024 or 1 000 000 000 000 000 000 000 000.
Adopted in 1991, it comes from the Greekὀκτώ (októ), meaning eight, because it is equal to 10008.
As of 2008, yotta- is the largest and last SI prefix to be confirmed.
Examples:
the mass of the Earth is 5973.6 Yg (although as large mass units are based on the tonne, this would be more likely expressed as 5.97 Zt).
the total power output of the Sun is approximately 383 YW.
the total data storage capacity produced by humans each year is expected to surpass 1 YB by 2013 1. In this context the prefix Y means 280 or ~ 1.21 x 1024.
the universe is estimated to be 880 Ym in diameter.