A physical quantity is defined either by specifying its measurement method or by stating how it is calculated from other measurements. For example, consider a metallic cube. We might define its mass and dimensions by specifying methods for measuring them, such as using a weighing machine and a meter scale. Then, we could define the volume by stating that it is the cube of its side, and we could calculate the density as the mass divided by the volume.

Measurements of physical quantities are expressed in terms of units, which are standardized values. For instance, the length of the side of a cube, which is a physical quantity, can be expressed in units of meters or centimeters, depending upon the size of the cube.

There are mainly two systems of units in use. The first is the metric system, which uses SI units, and the second is the customary or imperial system, which uses English units. SI is the international abbreviation for the French Système International d'Unités. It is the most widely used unit system globally and is considered the standard system by scientists and mathematicians.

This text is adapted from Openstax, University Physics Volume 1, Section 1.2: Units and Standards.