"A unit of measurement is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity". Example: A Length is a physical quantity. The metre is a unit of length that represents a definite predetermined length. 20 metres (or 20 m), we means 20 times the definite predetermined length called "metre". International System of Units (SI), the modern form of the metric system is now the global standard for Units of Measurement. The article Unit of Measurement lists the units of measurement that are prominently used everyday through out the globe. The SI units for the seven fundamental measurable qualities are:

Metre: The metre is the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 second.

Kilogram: The kilogram is the unit of mass equal to the mass of the international prototype of kilogram.

Second: The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levers (F=4, mF=0 to F=3, mF=0) of the ground state of the cesium 133 atom.

Ampere: The ampere is the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2x10-7 Newton per meter of length

Kelvin: The Kelvin, unit of thermodynamic temperature is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

Mole: The mole is the mount of substance of a system which contains as many elementary entities as there are atoms in .012 kg of carbon 12 (about 6.022x1023 atoms). When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

Candela: The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540x1012 Hz and that has a radiant intensity in that direction of 1/683 watt per steradian.

Measurement is the assignment of a number to a characteristic of an object or event, which can be compared with other objects or events. Thus, Measurement is a process of determining how large or small a physical quantity is as compared to a basic reference quantity of the same kind. A unit of measurement is a standardized quantity of a physical property, used as a factor to express occurring quantities of that property. As described in the history, Units of measurement were among the earliest tools invented by humans.
Unit of Measurement - Important Units of Measurement: