Unit of Measurement
Definition1
측정 단위 or simply 단위 refers to the standard used to express the magnitude of a physical quantity.
Description
It is the same as weights and measures.
In the past, measurements were based on a physical standard called a 원기. For example, to define the meter one would make a particular rod and declare “the length of this rod is 1 meter,” or to define the kilogram one would make a particular weight and declare “the mass of this weight is 1 kilogram.” Such methods have issues of accuracy and reliability: objects can undergo slight deformation or damage over time, and exact replication is difficult.
Therefore, in modern practice definitions based on natural constants are used so that the same value is obtained regardless of who, when, or where the measurement is made. In other words, units are defined by invariant physical phenomena rather than specific objects. For example, the current definition of the meter is the distance light travels in vacuum in a specified time. More precisely:
The distance light travels in vacuum during $\dfrac{1}{299\ 792\ 458}$ seconds is defined as $1$meter.
International System of Units (SI)
The standard system of units most widely used worldwide in modern science and engineering is the International System of Units (SI). The SI defines seven base units $\mathrm{s}$, $\mathrm{m}$, $\mathrm{kg}$, $\mathrm{A}$, $\mathrm{K}$, $\mathrm{mol}$, $\mathrm{cd}$, and expresses derived units such as speed ($\mathrm{m/s}$), force ($\mathrm{N} = \mathrm{kg \cdotp m/s^{2}}$), and energy ($\mathrm{J} = \mathrm{N \cdotp m}$) by combinations (multiplication and division) of these base units.