The standard amount of a physical quantity chosen to measure the physical quantity of the same kind is called a physical unit.
The essential requirement of a physical unit are as-
(I) It should be of suitable size.
(II) It should be easily accessible.
(III) It should not vary with time.
(IV) It should be easily reproductible.
(V) It should not depend on physical conditions like pressure, volume, temperature, etc.
Physical unit can be classified into two ways.
1. Fundamental Unit
The physical units which can neither be derived from one another, nor they can be further resolved into more simpler units are called fundamental units.
Examples:- Mass, length, time, etc.
2. Derived Unit
The units of measurement of all other physical quantities which can be obtained from fundamental units are called derived unit.