Differences

Zombie(Defunct) process

• On Unix and Unix-like computer operating systems, a zombie process or defunct process is a process that has completed execution but still has an entry in the process table.
• This entry is still needed to allow the parent process to read its child’s exit status.
• You cannot kill zombie process directly because it is already dead.
• To kill the zombie process, you should kill the parent process. However if the parent process is init(which is 1), then only thing you can do is reboot.
• It takes very tiny memory(description info) but will take process ID which is limited. So, it is better not to have zombie process.

Orphan process

• An orphan process is a computer process whose parent process has finished or terminated, though it remains running itself.
• parent process ID is changed to init which is 1.
• It is possible to find the orphan process and kill by kill -9 pid command.
• It still takes resources, and having too many orphan process will overload init process. It is better not to have many orphan process.