It is a system of International System of Units of measurement used widely around the world.
The value of a fundamental constant is expressed as the product of a number and a unit.
There are three core SI units: MKS (meter, kelvin, and seconds); FPS (foot, kelvin, and seconds); and CGS (centimeter, kelvin, and seconds).
Physical quantities are grouped into seven categories: mass (kilogram), length (meter m), time (second), temperature (kelvin), amount of substance (mole), electric current (ampere), and luminous intensity (candela).
Non-SI unit:
The terms are not dependent on the fundamental units such as M.K.S., F.P.S., and C.G.S. system.
The physical units that do not come under SI units are velocity, force, momentum, and magnetic field.
Non-SI units also include the direction and magnitude of the quantity.