Absolute + Relative Measurements (Design Dictionary)

Absolute and relative measurements are used within typography (font design).

Absolute measurements

These are easy to understand because they are measurements with a fixed value. This could be like mm is a defined increment of cm. However typography doesn't use mm or cm, it uses points and picas. These are typographic measurements with fixed values so can be expressed in finite terms and can't change

Relative measurements

These have many values, like character spacing. Character spacing is linked to the typesize meaning that it is defined by relative measurements, because if you increased the typesize, the character spacing would also increase.

For example, the basic building block of typographical characters is em. When the typesize is set at 70pt it has an em of 70pt, because it it to do with the scale of the characters.