Installing custom ROMs: since most Android systems don't have a ROM, isn't this a misnomer?
Old things tend to stick around. In everyday things (toasters, microwaves etc) there is some kind of software to control the device. That software is installed by the manufacturer and is usually hard to change, in practice read-only. While you can flash the system partition on android, with some electronic devices you needed to and still need to plug in some specific pins to some manufacturer application on a pc.

Nowadays ROM refers to the software by the manufacturer that drives the device, such as the /boot and /system partitions.