When a process exits (normally or abnormally), it enters a state known as “zombie”, which in top appears as "Z". Its process ID stays in the process table until its parent waits on or "reaps" it. Under normal circumstances, when the parent process fully expects its child processes to exit, it sets up a signal handler for SIGCHLD so that, when the signal is sent (upon a child process's exit), the parent process then reaps it at its convenience.
As long as the parent hasn't called wait(), the system needs to keep the dead child in the global process list, because that's the only place where the process ID is stored. The purpose of the "zombies" is really just for the system to remember the process ID, so that it can inform the parent process about it on request.
If the parent "forgets" to collect on its children, then the zombie will stay undead forever. If the parent itself dies, then "init" (the system process with the ID 1) will take over fostership over its children and catch up on the neglected parental duties. If the init process is stalled, then you have much bigger problem than child processes not being reaped. In fact, a crashed init process will usually cause a kernel panic.
