A zombie VM is a virtual machine that is not functioning properly in a software environment, but continues to operate, sucking up resources and providing no value.
Techopedia explains Zombie VM
The term "zombie VM" is sometimes used interchangeably with the term "orphaned VM," which is a machine that has been disconnected from its host. However, with a zombie VM, there is the implication that the VM is still actively using resources since it is still connected to its host. Along with incorrectly configured VMs, systems administrators also look for disconnected or otherwise useless VMDK files, the files held on these systems, to try to clean up and prevent what is often called "VM sprawl." VM sprawl happens when more VMs are deployed or created than are necessary. This occurs largely because of inefficiencies in building out systems and because of changes in systems over time, where, without rigorous analysis and constant management, resources become tied up in obsolete configurations.