They never died, they just got infected and mutated / grew fungal bits. Zombies are one of the many types of undead, ie dead that, for some unnatural and often sinister cause, have risen up again. In that sense, the enemies in the classic REs could be labelled zombies. Sure, it's an infection that causes it, but they still die and then somehow get reanimated.
Edit: Alternatively, you could just refer to zombies as formerly independent beings that now have no higher neurological functions anymore, in that their actions are effectively out of their control. That, however, is very hard to verify.