Zombie is a defunct child process in Unix that still has entry in process table.
Sometimes a child process is terminated in Unix, but the parent process still waits on it.
A Zombie process is different from an Orphan process. An orphan process is a child process whose parent process had died. Once a process is orphan it is adopted by init process. So effectively it is not an orphan.
Therefore if a process exits without cleaning its child processes, they do not become Zombie. Instead init process adopts these child processes.
Zombie processes are the ones that are not yet adopted by init process.