A physical phenomenon that can be quantified is a physical quantity.
It has any numeric value and unit.
It is the fixed amount of a physical quantity which is used as a standard of measurement.
Fundamental / Base quantity
Those quantities for which units are defined by international organizations are called fundamental quantities.
Those quantities for which units are not defined by international organizations are called derived quantities.
It is the comparison of an unknown quantity with a standard value.
System of Units
A set of fundamental and derived units is system of units.