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Mega is a unit prefix in metric systems of units denoting a factor of one million (10^{6} or 1000000). It has the unit symbol M. It was confirmed for use in the International System of Units (SI) in 1960. Mega comes from Ancient Greek: μέγας, romanized: mégas, lit. 'great'.^{[1]}
When units occur in exponentiation, such as in square and cubic forms, any multiplesprefix is considered part of the unit, and thus included in the exponentiation.
In some fields of computing, mega may sometimes denote 1,048,576 (2^{20}) of information units, for example, a megabyte, a megaword, but denotes 1000000 (10^{6}) units of other quantities, for example, transfer rates: 1megabit/s = 1000000bit/s. The prefix mebi has been suggested as a prefix for 2^{20} to avoid ambiguity.
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]} 

Origin: A borrowing from Greek. Etymon: Greek μεγα. ... Forming scientific and technical terms with the sense ‘very large’, ‘comparatively large’, or (esp. in Pathol.) ‘abnormally large’, often having correlatives beginning micro, and sometimes also synonyms beginning macro.
Look up mega in Wiktionary, the free dictionary. 