The metric system is a system of measurement in which each unit is named by combining a prefix denoting a power of 10 with the name of a base unit. For example, the centi- prefix represents 10-2; thus, a centimetre is 1/100 the length of a metre.

The base units are as follows:

Concept measured Name Symbol Definition
Time Second s The duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom at rest at 0K1
Distance Metre (spelt meter in America m The length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.2
Area Are a 100 square metres
Volume Litre (spelt liter in America l (lowercase L) Equal to 1dm3
Mass Kilogram kg The mass of a lump of metal in Paris after being cleaned in a certain way3
Temperature Kelvin K The fraction 1/273.16 of the thermodynamic temperature of the triple point of water4 (Easier definition: 1/100 of the difference in temperature between the freezing and boiling points of water at standard pressure)
Electric current Ampère A That constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 x 10–7 newton per metre of length5 (Easier definition: 96,500 electrons per second)
Substance Mole mol The amount of substance of a system which contains as many elementary entities as there are atoms in 12g of carbon 126
Luminous intensity Candela cd The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 Hz and that has a radiant intensity in that direction of 1/683 W per steradian6

The system has plenty of other units as well, but they are all defined in terms of the above; for example, the joule is defined as kg m2 s-2.

Note that the base unit of mass, unlike the others, has a prefix; namely kilo-. When the metric system was originally devised, the base unit of mass was called the grave, and was equal in magnitude to what is today called the kilogram. Gram was a slang term for what was then called the milligrave. However, grave also meant count; after the French Revolution, aristocracy was rather unpopular, and so the term was dropped in favour of kilogram.

Under the SI system, the are and litre are deprecated, with area and volume given in square metres and cubic metres respectfully. However, litres and hectares are commonly used in everyday life due to greater convenience..

Prefixes are denoted by, in all but one case, a single letter placed immediately before the symbol for the base unit. Each represents come power of 10. It is important to always use the prefix symbol in its correct case - there is a big difference between mm and Mm.

Prefix Symbol Magnitude
yocto- y 10-24
zepto- z 10-21
atto- a 10-18
femto- f 10-15
pico- p 10-12
nano- n 10-9
micro- µ 10-6
milli- m 10-3
centi- c 10-2
deci- d 10-1
deca- da 101
hecto- h 102
kilo- k 103
mega- M 106
giga- G 109
tera- T 1012
peta- P 1015
exa- E 1018
zetta- Z 1021
yotta- Y 1024

These prefixes can be added to any base unit you care to name (though note that in the case of mass, the prefix is applied to the gram and not kilogram). Thus, for example, 100 metres is a hectometre (1 hm), and one millionth of a joule is a microjoule (µJ). They can even be used for non-metric, non-SI units of measurement; for example, the American telecommunications industry traditionally measured wires in thousands of feet, and so it was only a matter of time before they started referring to kilofeet.

### References

1. http://www.bipm.org/en/si/si_brochure/chapter2/2-1/second.html
2. http://www.bipm.org/en/si/si_brochure/chapter2/2-1/metre.html
3. http://www.bipm.org/en/si/si_brochure/chapter2/2-1/kilogram.html
4. http://www.bipm.org/en/si/si_brochure/chapter2/2-1/kelvin.html
5. http://www.bipm.org/en/si/si_brochure/chapter2/2-1/ampere.html