cm
The certified centimeter: 96px divided by 2.54, not always a centimeter.
One cm is worth, by definition of the standard, 96px/2.54 — about 37.8 CSS pixels. The system's anchor is not the meter at the Sèvres bureau, it is the 96-pixel inch: the CSS centimeter is an imperial unit disguised as metric.
Like every physical absolute, its calling is paper: a business card of 8.5cm by 5.4cm leaves the printer at the agreed size. On screen, use it knowing it is an exchange rate, not a measurement.
.cartao {
width: 8.5cm;
height: 5.4cm;
}