The International System of Units (abbreviated SI from French: Le Système international d'unités) is the modern form of the metric system and is the world's most widely used system of measurement, used in both everyday commerce and science. It comprises a coherent system of units of measurement built around seven base units, 22 named and an indeterminate number of unnamed coherent derived units, and a set of prefixes that act as decimal-based multipliers.
The standards, published in 1960 as the result of an initiative started in 1948, are based on the metre–kilogram–second (MKS) system, rather than the centimetre–gram–second (CGS) system, which, in turn, had several variants. The SI has been declared to be an evolving system; thus prefixes and units are created and unit definitions are modified through international agreement as the technology of measurement progresses, and as the precision of measurements improves.
Uniformitarianism is the assumption that the same natural laws and processes that operate in the universe now have always operated in the universe in the past and apply everywhere in the universe. It has included the gradualistic concept that "the present is the key to the past" and is functioning at the same rates. Uniformitarianism has been a key principle of geology and virtually all fields of science, but naturalism's modern geologists, while accepting that geology has occurred across deep time, no longer hold to a strict gradualism.
Uniformitarianism was formulated by British naturalists in the late 18th century, starting with the work of the geologist James Hutton, which was refined by John Playfair and popularised by Charles Lyell's Principles of Geology in 1830. The term uniformitarianism was coined by William Whewell, who also coined the term catastrophism for the idea that Earth was shaped by a series of sudden, short-lived, violent events.
A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of measurement.
For example, length is a physical quantity. The metre is a unit of length that represents a definite predetermined length. When we say 10 metres (or 10 m), we actually mean 10 times the definite predetermined length called "metre".
A metric prefix or SI prefix is a unit prefix that precedes a basic unit of measure to indicate a decadic multiple or fraction of the unit. Each prefix has a unique symbol that is prepended to the unit symbol. The prefix kilo-, for example, may be added to gram to indicate multiplication by one thousand; one kilogram is equal to one thousand grams. The prefix centi-, likewise, may be added to metre to indicate division by one hundred; one centimetre is equal to one hundredth of a metre.
Decimal multiplicative prefixes have been a feature of all forms of the metric system with many dating back to the system's introduction in the 1790s. Metric prefixes have even been pre-pended to non-metric units. Today the prefixes are standardized for use in the International System of Units (SI) by the International Bureau of Weights and Measures in resolutions dating from 1960 to 1991.
A system of measurement is a set of units which can be used to specify anything which can be measured and were historically important, regulated and defined because of trade and internal commerce. In modern systems of measurement, some quantities are designated as fundamental units, meaning all other needed units can be derived from them, whereas in the early and most historic eras, the units were given by fiat (see statutory law) by the ruling entities and were not necessarily well inter-related or self-consistent.