Visual Display Unit (VDU) displays images that are generated by a computer. The term VDU is used synonymously with the monitor, but it can also refer to other different kinds of display including a digital projector. VDU may be peripheral devices or integrated with the other components. For example, the iMac uses an all-in-one device, in which both the computer and screen are built into a single unit.
Early Visual Display Units were primarily CRT (cathode ray tube) displays and had a diagonal size of 13 inches. During the 1990s, 15″ and 17″ displays size becomes standard, but some manufacturers began producing 20″ displays in size. At the turn of the century, the flat displays screens became more common, CRT displays were hard to find. Nowadays, it is common for computers to come with VDUs that are 20″ to 30″ in size.