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A zombie process is a process in its terminated state. This usually happens in a program that has parent-child functions. After a child function has finished execution, it sends an exit status to its parent function. Until the parent function receives and acknowledges the message, the child function remains in a “zombie” state, meaning it has executed but not exited.
A zombie process is also known as a defunct process.
A process in Unix or Unix-like operating systems becomes a zombie process when it has completed execution but one or some of its entries are still in the process table. If a process is ended by an "exit" call, all memory associated with it is reallocated to a new process; in this way, the system saves memory. But the process’ entry in the process table remains until the parent process acknowledges its execution, after which it is removed. The time between the execution and the acknowledgment of the process is the period when the process is in a zombie state.