The International System of Units is currently the generally accepted system of units in the physical sciences.
The international abbreviation of the name is SI (from the French Le Système International d'Unités).
The SI has four major components.
Seven defining constants (or reference constants) with exact values. These constants may be universal constants that arise from fundamental physical laws (Planck's constant h, Boltzmann's constant k, speed of light c), be connected to natural phenomena (hyperfine transition of a cesium atom ΔνCs, charge on a proton e), or have evolved from previous definitions of the base units (Avogadro's constant NA, luminous efficacy of green light Kcd).
Seven well-defined, dimensionally independent, base units that are assumed irreducible by convention (second, meter, kilogram, ampère, kelvin, mole, and candela).
A large number of derived units formed by combining base units according to the algebraic relations of the corresponding quantities (some of which are assigned special names and symbols and which themselves can be further combined to form even more derived units).
The derived units are coherent in the sense that they are all mutually related only by the rules of multiplication and division with no numerical factor other than 1 needed.
The derived units are also complete in the sense that one and only one unit exists for every defined physical quantity. Although it is possible to express many units in more than one way, they are all equivalent. (The converse statement is not necessarily true, however. Some units are used for more than one physical quantity.)
Twenty-four currently agreed upon prefixes that can be attached to any of the base units or derived units with special names creating multiples and division as needed. (The exception to this rule is the kilogram, which is already itself a multiple of the gram. In this case, prefixes should be added to the word gram.)
The first three named multiples are the first three powers of ten (101, 102, 103). Subsequent named multiples are larger than the previous named multiple by three orders of magnitude (106, 109, 1012, … ).
The first three named divisions are the first three negative powers of ten (10−1, 10−2, 10−3). Subsequent named divisions are smaller than the previous named division by three orders of magnitude (10−6, 10−9, 10−12, … ).
Other scientific, traditional, and practical units and unit systems are still in use and are still useful.
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