`text(Unit)`
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.

`text(Fundamental or Base Units)`
The units for the fundamental or base quantities are called fundamental or base units.

`text(Fundamental Quantities)`
The quantities that do not depend on any other physical quantity for their measurement are known as fundamental quantities.

`text(Derived 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.
Simply we can say that derived units are products of powers of base units.

`text(System of Units)`
A complete set of these units, both the base units and derived units, is known as the 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 (or British) 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 :
`text(CGS-)` centimetre, gram and second
`text(FPS-)` foot, pound and second
`text(MKS-)` metre, kilogram and second

`text(SI System)`
• The system of units which is at present internationally accepted for measurement is the Système Internationaled’ Unites (French for International System of Units), abbreviated as SI.
• In SI, there are seven base units as given in .Table.
`text(Note)`
When mole is used, the elementary entities must be specified. These entities may be atoms, molecules, ions, electrons, other particles or specified groups of such particles.
• Besides the seven base units, there are two more units - (a) radian (b) steradian

• The unit radian is defined for plane angle as the ratio of length of arc to the radius r.
Plane angle `d theta=(ds)/r`
• Symbol of radian = `text(rad)`

• The unit steradian is defined for solid angle as the ratio of intercepted area of spherical surface to the square of radius r.
Solid angle `dOmega=(dA)/r^2`
• Symbol of steradian = `text(sr)`