(1) Virtual machine. A simulated computer in that it runs on a host computer but behaves as if it were a separate computer. The Java virtual machine works as a self-contained operating environment to run Java applications and applets. (2) Virtual memory. The use of a disk partition or a file on disk to provide the facilities usually provided by RAM. The virtual-memory manager in Mac OS X provides either a 32-bit or 64-bit protected address space for each task (depending on the options used to build the task) and facilitates efficient sharing of that address space.