Built-in computer memory that can be read but not written to. ROM contains the programming that allows your computer to boot up each time you turn it on, and it contains essential system programs that neither you or the computer can erase. Unlike a computer's random access memory (RAM), ROM does not lose its data when the computer power is turned off. It is sustained by a small, long-life battery. (If you ever run the hardware setup procedure on your computer, you are in effect writing to ROM.)
ROM can also refer to Rough Order of Magnitude, referring to the amount of time it takes to accomplish a given task.