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Examples include number of particles and population size. In mathematics, the "number of elements" in a set is termed cardinality. Countable nouns is a related linguistics concept. Counting numbers, such as number of bits, can be compounded with units of frequency (inverse second) to derive units of count rate, such as bits per second.
In physics, a dimensionless physical constant is a physical constant that is dimensionless, i.e. a pure number having no units attached and having a numerical value that is independent of whatever system of units may be used. [1]
Definition Field of application Basic reproduction number: number of infections caused on average by an infectious individual over entire infectious period: epidemiology: Body fat percentage: total mass of fat divided by total body mass, multiplied by 100: biology Kt/V: Kt/V
While usually sub-second units are represented with SI prefixes on the second (e.g. milliseconds), this system can be extrapolated further, such that a "Third" would mean 1 ⁄ 60 of a second (16.7 milliseconds), and a "Fourth" would mean 1 ⁄ 60 of a third (278 microseconds), etc. These units are occasionally used in astronomy to denote angles.
The SI units are defined by declaring that seven defining constants [1]: 125–129 have certain exact numerical values when expressed in terms of their SI units. The realisation of the definition of a unit is the procedure by which the definition may be used to establish the value and associated uncertainty of a quantity of the same kind as the ...
A base unit of measurement (also referred to as a base unit or fundamental unit) is a unit of measurement adopted for a base quantity.A base quantity is one of a conventionally chosen subset of physical quantities, where no quantity in the subset can be expressed in terms of the others.
In physics, natural unit systems are measurement systems for which selected physical constants have been set to 1 through nondimensionalization of physical units.For example, the speed of light c may be set to 1, and it may then be omitted, equating mass and energy directly E = m rather than using c as a conversion factor in the typical mass–energy equivalence equation E = mc 2.
The first group of metric units are those that are at present defined as units within the International System of Units (SI). In its most restrictive interpretation, this is what may be meant when the term metric unit is used. The unit one (1) is the unit of a quantity of dimension one. It is the neutral element of any system of units. [2]