Zombie process is an inactive computer process.
A zombie process or defunct process is a process that has completed execution but still has an entry in the process table, allowing the process that started it to read its exit status.
* A process can be sleeping in kernel code. Usually that's because of faulty hardware or a badly written driver- or maybe a little of both. A device that isn't set to the interrupt the driver thinks it is can cause this, for example- the driver is waiting for something its never going to get. The process doesn't ignore your signal- it just never gets it.
A zombie process doesn't react to signals because it's not really a process at all- it's just what's left over after it died. What's supposed to happen is that its parent process was to issue a "wait()" to collect the information about its exit. If the parent doesn't (programming error or just bad programming), you get a zombie. The zombie will go away if its parent dies- it will be "adopted" by init which will do the wait()- so if you see one hanging about, check its parent; if it is init, it will be gone soon, if not the only recourse is to kill the parent..which you may or may not want to do.
Finally, a process that is being traced (by a debugger, for example) won't react to the KILL either.

So how do I find out zombie process?
# top
or
# ps aux | awk '{ print $8 " " $2 }' | grep -w Z
How do I kill zombie process?

You cannot kill zombies, as they are already dead. But if you have too many zombies then kill parent process or restart service.

You can kill zombie process using PID obtained from any one of the above command. For example kill zombie proces having PID 4104:
# kill -9 4104

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