Virtualization can be termed as a virtual version of any object such as Operating System, Hard disk, Memory, Networks, Storage Devices and so on; can be Created Logically on a Physical Hardware using Server Virtualization Products like Citrix XenServer, VMware, and Microsoft Hyper-v.
What can be virtualized?
Both Hardware and Software can be virtualized
Few Examples:
Memory, Hard Disk, CPU, Network Card, I/O cards, Servers, Desktops, Applications, LAN and more.