Definition - What does Overlay Keyboard mean?
An overlay keyboard refers to the redefining of function of keys on a keyboard by customizing it within an operating system. This can be done by reconfiguring the functionality of each and every key of a keyboard or by installing software that helps in modification of the internal function of each keystroke. Transliteration is one such example, where a standard English keyboard can be used for typing a different language using a completely different character set.
Techopedia explains Overlay Keyboard
Overlay keyboards are highly configurable, and can be used in for a variety of specialized tasks. An overlay keyboard extends the use of a standard keyboard. It can be used for various purposes and can aid in the performance of tasks where regular keyboards may take too much time or effort. Each key function can be remapped depending of the type of use. The keys may visually change, or they may remain visually the same but the command they send to the back end is changed.
Overlay keyboards are widely used in shopping centers at checkout counters where a few keys are used to enter different types of item data.