Zetta (symbol Z) is a prefix in the International System of Units (SI) denoting a factor of 10^{21} or 1000000000000000000000.
Added to the SI in 1991, it is derived from the French prefix septo, meaning seven, because it is equal to 1000^{7}.^{[1]}
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 now obsolete.
1000^{m}  10^{n}  Prefix  Symbol  Since^{[1]}  Short scale  Long scale  Decimal 

1000^{8}  10^{24}  yotta  Y  1991  Septillion  Quadrillion  1000000000000000000000000 
1000^{7}  10^{21}  zetta  Z  1991  Sextillion  Trilliard  1000000000000000000000 
1000^{6}  10^{18}  exa  E  1975  Quintillion  Trillion  1000000000000000000 
1000^{5}  10^{15}  peta  P  1975  Quadrillion  Billiard  1000000000000000 
1000^{4}  10^{12}  tera  T  1960  Trillion  Billion  1000000000000 
1000^{3}  10^{9}  giga  G  1960  Billion  Milliard  1000000000 
1000^{2}  10^{6}  mega  M  1960  Million  1000000  
1000^{1}  10^{3}  kilo  k  1795  Thousand  1000  
1000^{2⁄3}  10^{2}  hecto  h  1795  Hundred  100  
1000^{1⁄3}  10^{1}  deca  da  1795  Ten  10  
1000^{0}  10^{0}  (none)  (none)  NA  One  1  
1000^{−1⁄3}  10^{−1}  deci  d  1795  Tenth  0.1  
1000^{−2⁄3}  10^{−2}  centi  c  1795  Hundredth  0.01  
1000^{−1}  10^{−3}  milli  m  1795  Thousandth  0.001  
1000^{−2}  10^{−6}  micro  µ  1960^{[2]}  Millionth  0.000001  
1000^{−3}  10^{−9}  nano  n  1960  Billionth  Milliardth  0.000000001 
1000^{−4}  10^{−12}  pico  p  1960  Trillionth  Billionth  0.000000000001 
1000^{−5}  10^{−15}  femto  f  1964  Quadrillionth  Billiardth  0.000000000000001 
1000^{−6}  10^{−18}  atto  a  1964  Quintillionth  Trillionth  0.000000000000000001 
1000^{−7}  10^{−21}  zepto  z  1991  Sextillionth  Trilliardth  0.000000000000000000001 
1000^{−8}  10^{−24}  yocto  y  1991  Septillionth  Quadrillionth  0.000000000000000000000001 
Contents 
From Ancient Greek ἑπτά (hepta), for the seventh order of 10^{3}. The consonants pt are reduced to tt as in Italian, by analogy with peta; the letter z was added, as a start to running through the alphabet backwards.
zetta
