Learn in the textbookBasics of Semiconductor Memory — RAM, ROM, and Volatility
DefinitionROM is semiconductor memory that is read-only in principle and keeps its contents even when the power is turned off.
ExplanationContents written at manufacture or a similar stage are read out and used, and are not rewritten during normal use. This makes ROM suited to storing data that rarely changes, such as the program used to start up a device.
AnalogyIt is like a printed book. You can read it again and again, but its contents are not rewritten, and the words do not fade while it sits on the shelf.
Memory hookIt stands for Read Only Memory, and the single word Only carries the meaning "just for reading." Both start with R, but the R in RAM is Random, a different word.