virtual memory

In computing, a technique whereby a portion of the computer backing storage, or external, memory is used as an extension of its immediate-access, or internal, memory. The contents of an area of the immediate-access memory are stored on, say, a hard disk while they are not needed, and brought back into main memory when required.

The process, called paging or segmentation, is controlled by the computer operating system and is hidden from the programmer, to whom the computer's internal memory appears larger than it really is. The technique can be successfully implemented only if very fast backing store is available, so that ‘pages’ of memory can be rapidly switched into and out of the immediate-access memory.