Define a unit. What are its required characteristics?

The unit is a standard of measurement that contains definite amount of an invariable and accessible quantity. Other physical quantities can be compared w.r.t unit of a physical system.

The required characteristics of unit are:

(i) It should not change with time, place and physical conditions.

(ii) It should be easily reproducible.

(iii) It should neither be too small nor too large.

(iv) It should be independent of all other units.