If the parent of a process finishes, before the process itself, then after finishing the process is typically attached to 'init' as parent

init() typically keeps calling wait() for zombie processes to get cleaned up

A zombie process remains zombie forever, as there is no way to clean it up

A zombie process occupies space in OS data structures

A process can become zombie if it finishes, but the parent has finished before it

A process becomes zombie when it's parent finishes

Zombie processes are harmless even if OS is up for long time

A process becomes zombie when it finishes, and remains zombie until parent calls wait() on it