Virtual memory ballooning is a computer memory reclamation technique used by a hypervisor to allow the physical host system to retrieve unused memory from certain guest virtual machines (VMs) and share it with others.

Memory ballooning is a memory management feature used in most virtualization platforms which allows a host system to artificially enlarge its pool of memory by taking advantage or reclaiming unused memory previously allocated to various virtual machines.

This is achieved through a balloon driver which is installed on the guest operating system which the hypervisor communicates with when it needs to reclaim memory through ballooning.

To check ballooning status on Linux host through below command