Define fundamental units.

Hint: SI system of weights and measurements has been adopted to specify the units for a physical quantity. These units are broadly categorized into
1. Fundamental units
2. Derived units
On these fundamental units, are the dimensional formula based. There can be a large number of derived units defining a particular physical but all of them can be derived from the fundamental unit.

Complete step by step solution:
A fundamental unit is adopted for measurement of base quantities.
Base quantities are those chosen standards on which all other physical quantities depend. These base quantities are listed below:
1. Mass
2. Length
3. Time
4. Current
5. Temperature
6. Amount of a substance
7. Luminous intensity
For each of these base quantities, a unit has been specified by the SI system. These seven units are called the fundamental units. These fundamental units describe every other physical quantity. The fundamental units are chosen by convention.
The seven fundamental units are as follows:


These fundamental units are adopted for measurement everywhere and hence are universal in nature.

Note:
The symbols of some fundamental units start with capital letters while for some units, they start with the small letters. So, we must keep a note on the convention to write these units as well. These fundamental units can combine in any physical relation to form the derived units. The dimensional formula for the physical quantities is also based on the fundamental units.