virtual memory

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Memory, often as simulated on a hard disk, that emulates RAM, allowing an application to operate as though the computer has more memory than it actually does.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun computing Memory that appears to be RAM but is being simulated (by a process called paging) on a hard disk; allows a computer to operate as if it had more memory than it actually does, but with some loss of performance

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (computer science) memory created by using the hard disk to simulate additional random-access memory; the addressable storage space available to the user of a computer system in which virtual addresses are mapped into real addresses