A Physical phenomenon that can be quantified.It has any numeric value and unit.
All measurable quantities are called physical quantities such as length, mass, time and temperature. A physical quantity possesses at least two characteristics in common. One is its numerical magnitude and the other is the unit in which it is measured.
For example, if the length of a student is 104 cm then 104 is its numerical magnitude and centimeter is the unit of measurement.
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.