(a) It is defined as the reference standard used to measure a physical quantity.
(b) (i) They are defined as the units of fundamental quantities. They are independent of each other and are expressed by writing the letter of the fundamental quantity in parentheses, e.g., Fundamental units of mass, length and time are [M], [L], [T] respectively.
(ii) They are defined as those units which can be derived from fundamental units. They are expressed by writing the symbols of derived quantity in a parentheses.
e.g., D.U. of velocity = [u]
Acceleration = [a]
Pressure = [P]
Work = [W] and so on