(Or "defunct process") A Unix process that has terminated (either because it has been killed by a signal or because it has called exit()) and whose parent process has not yet received notification of its termination by executing (some form of) the wait() system call.
A zombie process exists solely as a process table entry and consumes no other resources. This entry is retained to hold the child's exit status until the parent process wants to retrieve it. The parent can also be notified asynchronously via a signal of the child's termination.
Zombie processes can be seen in "ps" listings occasionally (with a status "Z" in some versions).
Compare orphan process.