Zombie

On Unix operating systems,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. In the term’s colorful Metaphor,the child process has died
but has not yet been reaped.

When a process ends,all of the memory and resources associated with
it are deallocated so they can be used by other processes. However,
the process’s entry in the process table remains. The parent is sent a
SIGCHLD signal indicating that a child has died; the handler for this
signal will typically execute the wait system call,which reads the
exit status and removes the zombie. The zombie’s process ID and entry
in the process table can then be reused. However,if a parent ignores
the SIGCHLD,the zombie will be left in the process table. In some
situations this may be desirable,for example if the parent creates
another child process it ensures that it will not be allocated the
same process ID.

资料来源：http://wiki.answers.com/Q/What_is_Zombie_Process_and_Orphan_Process