)Also referred to as 1RU, although RU is not technically correct.
U is the standard unit of measure for designating the vertical usable space, or height of racks (metal frame designed to hold hardware devices) and cabinets (enclosures with one or more doors). This unit of measurement refers to the space between shelves on a rack. 1U is equal to 1.75 inches. For example, a rack designated as 20U, has 20 rack spaces for equipment and has 35 (20 x 1.75.) inches of vertical usable space. Rack and cabinet spaces and the equipment which fit into them are all measured in U.
Image Source: Site Planning Guide for Entry-Level Servers