Femto (symbol f) is a unit prefix in the metric system denoting a factor of 10^{−15}. It was added to the International System of Units (SI) in 1964.^{[1]} It is derived from the Danish word femten, meaning "fifteen", which has a vaguely similar spelling to fermi, which was introduced earlier to mean a femtometer.
Examples of use:
The femtometre shares the unit symbol (fm) with the older nonSI unit fermi, to which it is equivalent. The fermi, named in honour of Enrico Fermi, is often encountered in nuclear physics.
Prefix  Base 10  Decimal  English word  Adoption^{[nb 1]}  Etymology  

Name  Symbol  Short scale  Long scale  Language  Derived word  
yotta  Y  10^{24}  1000000000000000000000000  septillion  quadrillion  1991  Greek  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 
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  Greek  eight^{[nb 2]} 

Look up femto in Wiktionary, the free dictionary. 