### The International System of Units: Prefixes Added to Base Unit Names

The SI measurement system has several specific prefixes (a letter, number or word that precedes another word and gives it another meaning) that prepend the basic units of measure to form new words, thus new units which indicate multiples or fractions of the basic units. So, these prefixes serve as specifiers or mnemonic devices, and they also help make very large and very small numbers easier to understand and work with. The prefixes in The International System of Units are decimal fractions and multiples of SI units and derived SI units.
Each prefix has a unique word and symbol that is added to the basic unit.

The group of prefixes is universal, simple and easy to use.
Prefixes cannot be combined.

There are seven base units in the SI system:

the kilogram (kg), for mass
the second (s), for time
the Kelvin (K), for temperature
the ampere (A), for electric current
the mole (mol), for the amount of a substance
the candela (cd), for luminous intensity
the meter (m), for distance

The centimeter–gram–second system of units (abbreviated CGS or cgs) is a variant of the metric system based on the centimeter as the unit of length, the gram as the unit of mass, and the second as the unit of time.

Notice how the kilogram is the only SI base unit with a prefix already incorporated as part of its name and symbol. So the kilogram is an odd exception. As multiple prefixes are not acceptable, the symbols for decimal multiples and submultiples of the unit of mass are composed by adding the SI prefix symbols to g, and the name gram. So, a gram is defined as one-thousandth of the SI base unit, the kilogram, or 1×10−3 kg.
Example: 10-3 kg = 1 g (1 gram) but not: 10-3 kg = 1 mkg (1 millikilogram)

“Kilo” is a prefix deriving from a Greek word which means thousand. Antoine Lavoisier’s research group initially adopted it in 1795 and introduced into the metric system in France with its establishment in 1799.
The prefix “milli-,“ likewise, may be added to meter to indicate division by one thousand; one millimeter is equal to one-thousandth of a meter. Milli comes from the Latin word mille, meaning “one thousand.”

“Mega” is metric system unit prefix which denotes (a multiple of or) the equivalence of one million. The word originates from the Greek word which means great.
“Micro” is a prefix that indicates one-millionth fraction.

Most of the words for the prefixes originate from ancient Greek or Latin words, and some come from Spanish (the prefix Pico), Danish (the prefixes Femto, Atto) words.
For more of the unit prefixes, please check the table below:

Prefix Base 1000 Base 10 Decimal English word Adoption
Name Symbol Short scale Long scale
yotta Y  10008  1024 1000000000000000000000000  septillion  quadrillion 1991
zetta Z  10007  1021 1000000000000000000000  sextillion  trilliard 1991
exa E  10006  1018 1000000000000000000  quintillion  trillion 1975
peta P  10005  1015 1000000000000000  quadrillion  billiard 1975
tera T  10004  1012 1000000000000  trillion  billion 1960
giga G  10003  109 1000000000  billion  milliard 1960
mega M  10002  106 1000000  million 1873
kilo k  10001  103 1000  thousand 1795
hecto h  10002/3  102 100  hundred 1795
deca da  10001/3  101 10  ten 1795
10000  100 1  one
deci d  1000−1/3  10−1 0.1  tenth 1795
centi c  1000−2/3  10−2 0.01  hundredth 1795
milli m  1000−1  10−3 0.001  thousandth 1795
micro μ  1000−2  10−6 0.000001  millionth 1873
nano n  1000−3  10−9 0.000000001  billionth  milliardth 1960
pico p  1000−4  10−12 0.000000000001  trillionth  billionth 1960
femto f  1000−5  10−15 0.000000000000001  quadrillionth  billiardth (Proposed) 1964
atto a  1000−6  10−18 0.000000000000000001  quintillionth  trillionth 1964
zepto z  1000−7  10−21 0.000000000000000000001  sextillionth  trilliardth 1991
yocto y  1000−8  10−24 0.000000000000000000000001  septillionth  quadrillionth 1991