The term zombie process comes from the Unix operating system. It takes its meaning from the common definition of zombie--an undead person. In Unix, the most common cause of a zombie is where the process has exited, but its parent process has not yet done a "wait" system call for it. The book-keeping information for the zombie has to be retained so it can eventually be supplied to the parent when it does "wait". Typically, zombies that exist for more than a short period of time indicate a bug somewhere. The kind of zombie mentioned in the first paragraph does not occur in Unix and similar systems.