Physical quantities : -The quantities like length, mass, time, temperature, area, volume and density, etc., which can be measured, are known as physical quantity.

Basic physical quantity : - Length, mass, time, electric current, temperature, luminous intensity and amount of substance are called physical quantity.

Derived physical quantity : -A physical quantity obtained by multiplying or dividing one basic physical quantity with another physical quantity, is known as a derived quantity.

Unit : - A unit is a quantity or dimension used as a standard of measurement.

Meter : -The meter is the length of the path travelled by light in vacuum during a time interval of 1 / 299 792 458 of a second.

Kilogram : -The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

Second : -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 cesium 133 atom.

Ampere : - The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 newton per meter of length.

Kelvin : - The Kelvin, unit of thermodynamic temperature, is the fraction 1 / 273.16 of the thermodynamic temperature of the triple point of water.

Mole : - (i) 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. (ii) 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.

Candela : - The candela is the luminous intensity, in a given direction, of a source, that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1 / 683 watt per steradian.