Terminology
Key terms and concepts from the International Vocabulary of Metrology (VIM) and the ISO 80000 & IEC 80000 series. Understanding these is essential for navigating the standard.
Foundation
The Seven Base Quantities
The ISQ is built on seven base quantities, each with its own dimension symbol and SI unit.
Every quantity in ISO 80000 can be expressed as a product of powers of these seven base dimensions.
Maxwell (1873)
The Quantity Equation
A quantity equals its numerical value {Q} multiplied by its unit [Q].
Example: length of rod = 1.234 · metre
Glossary
Key Terms
Definitions adapted from the VIM (JCGM 200) and ISO 80000-1.
Quantity
QA property of a phenomenon, body, or substance that can be expressed as a number multiplied by a unit. The fundamental concept of metrology — everything ISO 80000 defines is a quantity.
length, mass, time, electric current, temperature
Kind of quantity
A grouping of quantities that can be compared, added, and subtracted because they share the same dimension. Length, width, and wavelength are all the same kind (dimension L).
all length-type quantities share kind L
Base quantity
One of the seven quantities that form the foundation of the International System of Quantities (ISQ). All other quantities are derived from these.
length, mass, time, electric current, temperature, amount of substance, luminous intensity
Derived quantity
A quantity defined in terms of base quantities via a system of equations. Most quantities in ISO 80000 are derived.
velocity = length / time, density = mass / volume
Dimension
dim QAn expression of the dependence of a quantity on the base quantities as a product of powers of the base dimension symbols. Written as L, M, T, I, Θ, N, J.
dim(velocity) = L T⁻¹, dim(force) = L M T⁻²
Unit
A particular quantity, defined and adopted by convention, used as a standard to express quantities of the same kind. The SI defines the coherent system of units.
metre (m) for length, kilogram (kg) for mass, second (s) for time
Quantity value
{Q}The number multiplying the unit in a quantity value expression. A quantity equals its value times its unit: Q = {Q} · [Q] (Maxwell, 1873).
for "1.234 m", the value is 1.234 and the unit is metre
Measurement
The process of experimentally obtaining one or more quantity values that can reasonably be attributed to a quantity. Produces a Level 3 "quantity as measured."
using a ruler to measure the length of a rod
Entity
A phenomenon, body, or substance to which a quantity can be attributed. Quantities do not exist in isolation — they characterize entities.
a rod (entity) has length (quantity); an electron (entity) has mass (quantity)
Ordinal quantity
A quantity defined by a conventional measurement procedure for which the total ordering can be established, but for which no algebraic operations are defined.
Rockwell C hardness, octane number — not covered by ISO 80000
For the full concept system hierarchy (kind → quantity → dedicated → measured), see:
Concept System