The standard amount of a physical quantity chosen to measure the physical quantity of the same kind is called a physical unit.
Desirable characteristics of a physical unit:
1. It should be well-defined.
2. It should be of convenient size, i.e., neither too small nor too large in comparison with the measurable physical quantity.
3. It should not change with time.
4. It should be easily reproducible.
5. It should be imperishable or indestructible
6. It should not be affected by the change in physical conditions such as pressure, temperature, etc.
7. It should be internationally acceptable.
8. It should be easily accessible.
Fundamental Units:
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. The units of fundamental quantities such as mass, length, etc. are fundamental units.
Derived Units:
All the other physical units which can be expressed in terms of the fundamental units are called derived units.