The International Bureau of Weights and Measures defines a magnitude as an attribute of a phenomenon, a body or substance that can be distinguished qualitatively and quantitatively determined, that is, you can assign different values as a result of a measurement or a list of measures. The physical magnitudes are measured using a pattern that has well defined that magnitude, and taking as unit the amount of that property held by the pattern object. For example, it is considered that the main pattern of length is the meter in the International System of Units.
The first definite magnitudes were related to the measurement of length, area, volume, mass, and length of time periods.
Many properties of matter are quantitative, i.e., are associated with the amount thereof and, therefore, with a number. When this number represents a magnitude measurement, its units must be specified. To say that a brick weighs 1.2 has no sense, but expressing the number with its units, 1.2 kilogram (kg) clearly specifies its weight.
In science the metric system of units, which was developed in France in the late eighteenth century, is used as a system of measures in many countries. In 1960 an international agreement was reached to use an adaptation of the metric system in scientific measurements. This system is called the International System of Units or simply IS.
An essential feature, which is the great advantage of the International System is that their units are based on fundamental physical phenomena. The only exception is the unit of mass,the kilogram, defined as “the mass of the international prototype of the kilogram”, a platinum-iridium cylinder stored in a safe at the International Bureau of Weights and Measures which is in Paris.
The International System consists of seven base (or fundamental) units expressing physical quantities, of which all others are derived. The table shows these base units and their symbols.
Just for your information, next are included the definitions of base magnitudes of the International System:
Length, meter (m): is the distance traveled by light in vacuum during 1/299,792,458 seconds. This pattern was established in 1983.
Mass, kilogram (kg): is the mass of a cylinder of platinum-iridium alloy deposited at the International Bureau of Weights and Measures. This pattern was established in 1887.
Time, second (s): is the duration of 9,192,631,770 periods corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 radiation. This pattern was established in 1967.
Temperature, kelvin (K): is the fraction 1/273.16 of the temperature of the triple point of water.
Amount of substance, mole (mol): is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12. The “elementary entities” must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.
Electric current intensity, ampere (A): the intensity of a constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section and located at a distance of one meter from each other in the vacuum would produce between these conductors a force equal to 2×10-7 newton per meter of length.
Luminous intensity, candela (cd): is the lighting unit in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 Hz and whose radiant intensity in that direction is 1/683 watt per steradian.