Electronically Erasable Read Only Memory
It's re-programmable (i.e. by re-flashing it).
edit: I should add that motherboard manufacturers could prevent this type of attack by "locking" the BIOS for flashing unless it was explicitly unlocked by changing a setting in the BIOS menu (some have this already, I believe). The problem at the moment is that the BIOS is writable at all times, even when the OS is running. This makes BIOS updates easier (i.e. you can make a Windows application that can do so, for example), but the problem is that this allows ANY process with Admin access to alter the BIOS as well.