What is a unit? What are fundamental units and derived units?

(N/A) unit is defined as the comparison of a physical quantity with a certain basic,internationally accepted reference standard.
- The measurement of any physical quantity is expressed by a numerical value accompanied by a unit.
- Although the number of physical quantities is very large,we only need a limited number of units to express all of them,as they are interrelated.
- The units used for fundamental or base quantities are called fundamental or base units (e.g.,$meter$,$kilogram$,$second$).
- Physical quantities that are expressed as a combination of fundamental quantities are called derived quantities,and their units are called derived units (e.g.,$speed = distance/time$,so the unit is $m/s$).