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The former Weights and Measures office in Seven Sisters, London Units of measurement, Palazzo della Ragione, Padua. A unit of measurement, or unit of measure, 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. [1]
The SI comprises a coherent system of units of measurement starting with seven base units, which are the second (symbol s, the unit of time), metre (m, length), kilogram (kg, mass), ampere (A, electric current), kelvin (K, thermodynamic temperature), mole (mol, amount of substance), and candela (cd, luminous intensity).
SI derived units are units of measurement derived from the seven SI base units specified by the International System of Units (SI). They can be expressed as a product (or ratio) of one or more of the base units, possibly scaled by an appropriate power of exponentiation (see: Buckingham π theorem).
In mathematics, the concept of quantity is an ancient one extending back to the time of Aristotle and earlier. Aristotle regarded quantity as a fundamental ontological and scientific category.
SI base units Name Symbol Measure Post-2019 formal definition [1] Historical origin / justification Dimension symbol; second: s time "The second, symbol s, is the SI unit of time.
The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m 2 ⋅s −3. [1] [2] [3] It is used to quantify the rate of energy transfer.
Dimensionless quantities, or quantities of dimension one, [1] are quantities implicitly defined in a manner that prevents their aggregation into units of measurement. [2] [3] Typically expressed as ratios that align with another system, these quantities do not necessitate explicitly defined units.
The kilogram (also spelled kilogramme [1]) is the base unit of mass in the International System of Units (SI), having the unit symbol kg. [1] The word "kilogram" is formed from the combination of the metric prefix kilo-(meaning one thousand) and gram; [2] it is colloquially shortened to "kilo" (plural "kilos").