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 1048576 (2^{20}) 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. In the case of 3½inch floppy disks, sizes were given in megabytes of 1000KB or 1024000 bits.^{[2]} The prefix mebi has been suggested as a prefix for 2^{20} to avoid ambiguity.
Prefix  Base 10  Decimal  Adoption ^{[nb 1]}  

Name  Symbol  
quetta  Q  10^{30}  1000000000000000000000000000000  2022^{[3]} 
ronna  R  10^{27}  1000000000000000000000000000  
yotta  Y  10^{24}  1000000000000000000000000  1991 
zetta  Z  10^{21}  1000000000000000000000  
exa  E  10^{18}  1000000000000000000  1975^{[4]} 
peta  P  10^{15}  1000000000000000  
tera  T  10^{12}  1000000000000  1960 
giga  G  10^{9}  1000000000  
mega  M  10^{6}  1000000  1873 
kilo  k  10^{3}  1000  1795 
hecto  h  10^{2}  100  
deca  da  10^{1}  10  
—  —  10^{0}  1  — 
deci  d  10^{−1}  0.1  1795 
centi  c  10^{−2}  0.01  
milli  m  10^{−3}  0.001  
micro  μ  10^{−6}  0.000001  1873 
nano  n  10^{−9}  0.000000001  1960 
pico  p  10^{−12}  0.000000000001  
femto  f  10^{−15}  0.000000000000001  1964 
atto  a  10^{−18}  0.000000000000000001  
zepto  z  10^{−21}  0.000000000000000000001  1991 
yocto  y  10^{−24}  0.000000000000000000000001  
ronto  r  10^{−27}  0.000000000000000000000000001  2022^{[3]} 
quecto  q  10^{−30}  0.000000000000000000000000000001  

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.