A rack unit or U is a unit of measure used to describe the height of equipment intended for mounting in a 19-inch rack or a 23-inch rack
(The 19-inch or 23-inch dimension refers to the width of the equipment
mounting frame in the rack i.e. the width of the equipment that can be
mounted inside the rack). One rack unit is 1.75 inches (44.45 mm) high.
The size of a piece of rack-mounted equipment is frequently described
as a number in "U". For example, one rack unit is often referred to as
"1U", 2 rack units as "2U" and so on.