A zombie process is a sub-process that finished (normally or otherwise), but it's not yet reaped by its parent (i.e.: its parent didn't collect its exit code). It can't be killed (because it's already dead, hence "zombie"), but it won't consume resources (CPU and memory). However, it is still using a scheduler process id; it's theoretically possible to starve a system of pids (therefore, preventing it to create new processes) launching dummy subprocesses and not reaping them.