Where does the term flashing come from?
ROM is a short for read-only memory. Nowadays, it is often a writable memory that is not written in normal operation. In past days, it used to be rather EPROM/EEPROM.

The term flashing probably comes from era of EPROM, which can be erased using a strong UV light. Erasure of whole memory is needed before EPROM is written. This is not the case of current flash memories, but the term “flashing” remains.