Definition
A type of computer memory chip that is manufactured blank and then permanently written with data one time, after which the stored information can be read but not changed. Once programmed, the contents are retained even when electrical power is removed.
Plain English
A memory chip that the manufacturer or technician writes information onto once, and after that the chip just holds onto that information forever and only lets you read it back.
Context Anchor
Seen in avionics and aircraft maintenance discussions about electronic units that store instructions, settings, or fixed data.
Derivation
Programmable means it can be written to once by the user rather than only at the factory. Read-Only means after that one writing, the data can be read but not rewritten. The combination distinguishes it from earlier ROM chips, which were fixed at manufacture, and from RAM, which loses its contents when power is removed.
Why Pilots Care
Preserves critical software and calibration data without risk of accidental alteration during flight operations.
Analogy
It is like a checklist printed on a card: once the card is made, you can read it whenever you need it, but you are not rewriting it during normal use.
Intuition Check
Read-only does not mean the memory can never be written under any condition. It means that during normal equipment operation, the stored information is read and treated as fixed.
Example Sentence 1
The avionics technician replaced the PROM in the navigation unit because the stored database needed to be updated to the latest revision.
Example Sentence 2
During an upgrade the avionics shop reprograms the programmable read-only memory chips with the latest software version.