Android ROM. (Android Read Only Memory) A file containing the executable instructions (a system image) of an Android OS and affiliated apps. The "stock ROM" comes installed on the phone or tablet, while a "custom ROM" comes from a third party.
Also, what is a ROM on Android?
The term ROM, which stands for Read Only Memory and really has very little to do with what a custom Android ROM actually is, can be confusing. A custom Android ROM refers to a phone's firmware, based on Google's Android platform. Users can install ROMs to change a device's appearance and behavior.
What is the difference between stock ROM and firmware?
A Stock ROM/Firmware is an official software that is designed by the manufacturer for a particular device. A Truly Stock ROM is one type that does not undergo any cosmetic/functional changes in the code by hardware manufacturer. E.g: Android running on Google pixel, Nexus phones.