mm
The metric system's millimeter, with the caveat that a screen is not paper.
One mm is a tenth of a centimeter — or, in the standard's internal arithmetic, 96px divided by 25.4. On printed paper the math agrees with the physical world; on screen, every absolute unit is converted via the reference pixel, and the monitor's millimeter rarely measures a millimeter on the glass.
It is the right choice in print stylesheets: @page margins, labels, forms that must match A4 paper — territory where the millimeter truly rules. On screen, prefer the units that were born there.
@page {
margin: 20mm 15mm;
}