Definition: viewable image
The diagonal image on a monitor's screen. The diagonal is always longer than the horizontal measurement. When CRTs were the norm, their sizes were often overstated. For example, a 17" CRT was generally between 15" and 15.8" diagonal. On the other hand, flat panels on laptops and desktop monitors have always been accurate, which is the actual "viewable image." A 23" flat panel is actually 23" diagonal. What a concept!