Kilo is a decimal unit prefix in the metric system denoting multiplication by one thousand (10^{3}). It is used in the International System of Units, where it has the symbol k, in lower case.
The prefix kilo is derived from the Greek word χίλιοι (chilioi), meaning "thousand".
In 19th century English it was sometimes spelled chilio, in line with a puristic opinion by Thomas Young.^{[1]}^{[2]} As an opponent of suggestions to introduce the metric system in Britain, he qualified the nomenclature adopted in France as barbarous.
By extension, currencies are also sometimes preceded by the prefix kilo:
For the kilobyte, a second definition has been in common use in some fields of computer science and information technology. It uses kilobyte to mean 2^{10} bytes (= 1024 bytes), because of the mathematical coincidence that 2^{10} is approximately 10^{3}. The reason for this application is that digital hardware and architectures natively use base 2 exponentiation, and not decimal systems. JEDEC memory standards still permit this definition, but acknowledge the correct SI usage.
NIST comments on the confusion caused by these contrasting definitions: "Faced with this reality, the IEEE Standards Board decided that IEEE standards will use the conventional, internationally adopted, definitions of the SI prefixes", instead of kilo for 1024.^{[3]} To address this conflict, a new set of binary prefixes has been introduced, which is based on powers of 2. Therefore, 1024 bytes are defined as one kibibyte (1 KiB).
When units occur in exponentiation, such as in square and cubic forms, any multiplier prefix is considered part of the unit, and thus included in the exponentiation.
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, monster^{[1]} 
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^{[nb 3]}  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 4]} 
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]} 
