Measuring things is as important an aspect of web design, as any other. The fact that we have at least 10 different measurement units in CSS speaks for itself.
Absolute units are a digital representation of actual measurements in the physical world. These units are not related to the size of the screen or its resolution. As a result, absolute length units are not as well suited for use on digital devices or when the resolution is not known. These units are more appropriate when you are designing for physical media such as print.

❶ cm (centimeters)
❷ mm (millimeters)
❸ in (inches)
❹ pc (picas)
❺ pt (points)
❻ px (pixels)