Introduction
Measurement of any physical quantity involves comparison with a certain basic, arbitrarily chosen, internationally accepted reference standard called unit. The result of a measurement of a physical quantity is expressed by a number (or numerical measure) accompanied by a unit.Although the number of physical quantities appears to be very large, we need only a limited number of units for expressing all the physical quantities, since they are interrelated with one another. The units for the fundamental or base quantities are called fundamental or base units.
The units of all other physical quantities can be expressed as combinations of the base units. Such units obtained for the derived quantities are called derived units. A complete set of these units, both the base units and derived units, is known as the system of units.
The International System of Units:
In earlier time scientists of different countries were using different systems of units for measurement. Three such systems, the CGS, the FPS system and the MKS system were in use extensively till recently.
The base units for length, mass and time in these systems were as follows
 In CGS system they were centimetre, gram and second respectively.
 In FPS system they were foot, pound and second respectively.
 In MKS system they were metre, kilogram and second respectively.
The SI, with standard scheme of symbols, units and abbreviations, was developed and recommended by General Conference on Weights and Measures in 1971 for international usage in scientific, technical, industrial and commercial work. Because SI units use decimal system, conversions within the system are quite simple and convenient. We shall follow the SI units.
In SI, there are seven base units. Besides the seven base units, there are two more units that are defined for,
 plane angle 'dÎ¸' as the ratio of length of arc 'ds' to the radius 'r'
 solid angle 'dÎ©' as the ratio of the intercepted area 'dA' of the spherical surface, described about the apex O as the centre, to the square of its radius 'r'.
Table SI Base Quantities and Units
Basic Quantity 
SI Units 

Name 
Symbol 
Definition 

Length 
metre 
m 
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. (1983) 
Mass 
kilogram 
kg 
The kilogram is equal to the mass of the international prototype of the kilogram (a platinumiridium alloy cylinder) kept at international Bureau of Weights and Measures, at Sevres, near Paris, France. (1889) 
Time 
second 
s 
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 cesium133 atom. (1967) 
Electric current 
ampere 
A 
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 2Ã—10^{â€“7} newton per metre of length. (1948) 
Thermo dynamic Temperature 
kelvin 
K 
The kelvin, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. (1967) 
Amount of substance 
mole 
mol 
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. (1971) 
Luminous intensity 
candela 
cd 
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540Ã—1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. (1979) 