Look up zepto in Wiktionary, the free dictionary. 
Zepto (unit symbol z) is a unit prefix in the metric system denoting a factor of 10^{−21}, i.e. 0.000000000000000000001.
Adopted into the International System of Units (SI) in 1991, it is derived from the Latin septem, seven, since it is equal to 1000^{−7}.
Examples of its use:
Prefix  Base 10  Decimal  English word  Adoption^{[nb 1]}  Etymology  

Name  Symbol  Short scale  Long scale  Language  Source word  
yotta  Y  10^{24}  1000000000000000000000000  septillion  quadrillion  1991  Latin  eight^{[nb 2]} 
zetta  Z  10^{21}  1000000000000000000000  sextillion  trilliard  1991  Latin  seven^{[nb 2]} 
exa  E  10^{18}  1000000000000000000  quintillion  trillion  1975  Greek  six 
peta  P  10^{15}  1000000000000000  quadrillion  billiard  1975  Greek  five^{[nb 2]} 
tera  T  10^{12}  1000000000000  trillion  billion  1960  Greek  four,^{[nb 2]} monster 
giga  G  10^{9}  1000000000  billion  milliard  1960  Greek  giant 
mega  M  10^{6}  1000000  million  1873  Greek  great  
kilo  k  10^{3}  1000  thousand  1795  Greek  thousand  
hecto  h  10^{2}  100  hundred  1795  Greek  hundred  
deca  da  10^{1}  10  ten  1795  Greek  ten  
10^{0}  1  one  –  
deci  d  10^{−1}  0.1  tenth  1795  Latin  ten  
centi  c  10^{−2}  0.01  hundredth  1795  Latin  hundred  
milli  m  10^{−3}  0.001  thousandth  1795  Latin  thousand  
micro  μ  10^{−6}  0.000001  millionth  1873  Greek  small  
nano  n  10^{−9}  0.000000001  billionth  milliardth  1960  Greek  dwarf 
pico  p  10^{−12}  0.000000000001  trillionth  billionth  1960  Spanish  peak, a little bit 
femto  f  10^{−15}  0.000000000000001  quadrillionth  billiardth  1964  Danish  fifteen, Fermi^{[nb 3]} 
atto  a  10^{−18}  0.000000000000000001  quintillionth  trillionth  1964  Danish  eighteen 
zepto  z  10^{−21}  0.000000000000000000001  sextillionth  trilliardth  1991  Latin  seven^{[nb 2]} 
yocto  y  10^{−24}  0.000000000000000000000001  septillionth  quadrillionth  1991  Latin  eight^{[nb 2]} 
