An EPROM, or erasable programmable read-only memory, is a type of computer memory chip that retains its data when its power supply is switched off. In other words, it is non-volatile. It is programmed by an electronic device that supplies higher voltages than those normally used in electronic circuits. Once programmed, an EPROM can be erased only by exposing it to strong ultraviolet light. EPROMs are easily recognisable by the transparent window in the top of the package, through which the silicon chip can be seen, and which admits UV light during erasing.
A programmed EPROM retains its data for about ten to twenty years and can be read an unlimited number of times. To prevent accidental erasure by sunlight, the erasing window must be kept covered. Old PC BIOS chips were often EPROMs, and the erasing window was often covered with a label containing the BIOS publisher's name, the BIOS revision, and a copyright notice.