Active Memory – “Amount of memory that is actively used, as estimated by VMkernel based on recently touched memory pages.”
Hypervisor estimate of memory actively being used in the VM's guest OS. The hypervisor does not communicate with the guest OS, so it does not know if any memory allocated to the VM is no longer needed
Detailed explanation

XorMon NG presents VM active memory in

• Cluster memory usage total graph
• VM TOP as a table
• vCenter and cluster totals under "Memory VMs Active" tab as graphs

vCenter active memory example