Originally, computer monitors were used for data processing while television receivers were used for entertainment. From the 1980s onwards, computers (and their monitors) have been used for both data processing and entertainment, while televisions have implemented some computer functionality. The common aspect ratio of televisions, and computer monitors, has changed from 4:3 to 16:10, to 16:9.
Modern computer monitors are easily interchangeable with conventional television sets. However, as computer monitors do not necessarily include components such as a television tuner and speakers, it may not be possible to use a computer monitor as a television without external components.
Early electronic computers were fitted with a panel of light bulbs where the state of each particular bulb would indicate the on/off state of a particular register bit inside the computer. This allowed the engineers operating the computer to monitor the internal state of the machine, so this panel of lights came to be known as the 'monitor'. As early monitors were only capable of displaying a very limited amount of information and were very transient, they were rarely considered for program output. Instead, a line printer was the primary output device, while the monitor was limited to keeping track of the program's operation.
As technology developed engineers realized that the output of a CRT display was more flexible than a panel of light bulbs and eventually, by giving control of what was displayed in the program itself, the monitor itself became a powerful output device in its own right.
Computer monitors were formerly known as visual display units (VDU), but this term had mostly fallen out of use by the 1990s.