Define the term standard unit.

Hint: For all scientific purposes, physical quantities are measured in their standard units. Standard units help in avoiding discrepancy and variations of the measurement done by different persons and provides for a common unit.

Complete answer:
Standard units are a set of units that have been designated as the units that have to be used for all scientific purposes universally by convention.
Using a standard unit helps in avoiding discrepancies in measurements that may arise due to different people measuring the same quantity but by using different units of their choice and convenience.
For example, in the United States, the mass of a body is often measured in pounds and in Europe it is measured in kilograms. However, according to the standard unit system, the standard unit of mass is the kilogram and hence, for scientific purposes the mass of a body should be measured in kilogram.
Therefore, it can also be said that standard units remain constant and do not change regardless of whosoever is using it in whatsoever place.
This also helps in the purpose that all people in the world interpret the same meaning of some physical measurement as the standard units for a physical measurement will convey a single meaning for the measurement, universally.

Note: There are also some other units other than the standard units that are used commonly for measurement because of their convenience or because the standard units are not adequate to measure the quantities for day to day life or commercial purposes. For example, the standard unit of power is Watt. However, another unit known as the Horsepower is also commonly used to measure power particularly in the automobile industry due to its convenience. Students should know of such other important quantities also.