The International System of Units (SI) defines seven base units^{[1]} for a set of physical quantities of measure, or dimensions, that are used to define all other SI units, known as SI derived units.
The set of SI basic units consists of the metre, kilogram, second, ampere, kelvin, mole, and candela, which are the units for length, mass, time, electrical current, temperature, quantity of substance, and luminous intensity, respectively.
The SI base quantities of measure form a set of linearly independent dimensions as required by dimensional analysis commonly employed in science and technology. However, in a given realization of these units they may well be interdependent, i.e., defined in terms of each other.^{[1]}
The names of all SI units are written in lowercase characters (e.g., meter, symbol: m), while the symbols of units named after persons are written with an initial capital letter (e.g., ampere, symbol: A).
Many other units, such as the litre, are not SI units but are nonSI units accepted for use with SI.
Contents 
Name  Symbol  Measure  Definition  Historical Origin / Justification 

metre  m  length  "The metre is the length of the path travelled by light in
vacuum during a time interval of 1/299 792 458 of a
second." 17th CGPM (1983, Resolution 1, CR, 97) 
^{1}⁄_{10,000,000} of the distance from the Earth's equator to the North Pole measured on the circumference through Paris. 
kilogram  kg  mass  "The kilogram is the unit of mass; it is equal to the mass of
the international prototype of the kilogram." 3rd CGPM (1901, CR, 70) 
The mass of one litre of water. A litre is one thousandth of a cubic metre. 
second  s  time  "The second is 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." 13th CGPM (1967/68, Resolution 1; CR, 103) "This definition refers to a caesium atom at rest at a temperature of 0 K." (Added by CIPM in 1997) 
The day is divided in 24 hours, each hour divided in 60
minutes, each minute divided in 60 seconds. A second is ^{1}⁄_{(24 × 60 × 60)} of the day 
ampere  A  electric current  "The ampere is that constant current which, if maintained in
two straight parallel conductors of infinite length, of negligible
circular crosssection, and placed 1 metre apart in vacuum, would
produce between these conductors a force equal to 2 × 10^{−7} newton per
metre of length." 9th CGPM (1948) 
The original "International Ampere" was defined electrochemically as the current required to deposit 1.118 milligrams of silver per second from a solution of silver nitrate. Compared to the SI ampere, the difference is 0.015%. 
kelvin  K  thermodynamic temperature  "The kelvin, unit of thermodynamic temperature, is the fraction
1/273.16 of the thermodynamic temperature of the triple point of
water." 13th CGPM (1967/68, Resolution 4; CR, 104) "This definition refers to water having the isotopic composition defined exactly by the following amount of substance ratios: 0.000 155 76 mole of ^{2}H per mole of ^{1}H, 0.000 379 9 mole of ^{17}O per mole of ^{16}O, and 0.002 005 2 mole of ^{18}O per mole of ^{16}O." (Added by CIPM in 2005) 
The Celsius scale: the Kelvin scale uses the degree Celsius for its unit increment, but is a thermodynamic scale (0 K is absolute zero). 
mole  mol  amount of substance  "1. The mole is the amount of substance of a system which
contains as many elementary entities as there are atoms in 0.012
kilogram of carbon 12; its symbol is “mol”. / 2. When the mole
is used, the elementary entities must be specified and may be
atoms, molecules, ions, electrons, other particles, or specified
groups of such particles." 14th CGPM (1971, Resolution 3; CR, 78) "In this definition, it is understood that unbound atoms of carbon 12, at rest and in their ground state, are referred to." (Added by CIPM in 1980) 
Atomic weight or molecular weight multiplied by the molar mass constant, 1 g/mol. 
candela  cd  luminous intensity  "The candela is the luminous intensity, in a given direction,
of a source that emits monochromatic radiation of frequency 540 × 10^{12} hertz and
that has a radiant intensity in that direction of 1/683 watt per
steradian." 16th CGPM (1979, Resolution 3; CR, 100) 
The candlepower, which is based on the light emitted from a burning candle of standard properties. 
There have been several modifications to the definitions of the base units, and additions of base units, since the Metre Convention in 1875. Since the redefinition of the metre in 1960, the kilogram is the only unit which is directly defined in terms of a physical artifact rather than a property of nature. However, the mole, the ampere and the candela are also linked through their definitions to the mass of this platinum–iridium cylinder stored in a vault near Paris. It has long been an objective of metrology to find a way to define the kilogram in terms of a fundamental constant, in the same way that the metre is now defined in terms of the speed of light.
The 21st General Conference on Weights and Measures (CGPM, 1999) placed these efforts on an official footing, and recommended "that national laboratories continue their efforts to refine experiments that link the unit of mass to fundamental or atomic constants with a view to a future redefinition of the kilogram." Two main possibilities have attracted attention: the Planck constant and the Avogadro constant.
In 2005, the International Committee for Weights and Measures (CIPM) approved the preparation of new definitions for the kilogram, the ampere and the kelvin, and noted the possibility of a new definition for the mole based on the Avogadro constant.^{[2]} The 23rd CGPM (2007) decided to postpone any legal change until the next General Conference in 2011.^{[3]}
