ROM - Read Only Memory
In theory, this form of memory can only be written to once, and is from that point on etched in stone - as it were. Realistically - very few forms of ROM are actually that fixed. It is more common these days to see EEPROM or Flash Memory in use.
ROM is commonly used to store low-level programs, for example the code to boot up a system like a PC BIOS. In the early days of computing, many programs were distributed on ROM Chips and even now, in a number of consoles - the cartridge system is exactly that.
The acronym was chosen, because it fits nicely with its counterpart RAM.