a system whereby addressable memory is extended beyond main storage through the use of secondary storage managed by system software in such a way that programs can treat all of the designated storage as addressable main storage.
Another possible storage option to consider would be virtual storage.
British Dictionary definitions for virtual storage
virtual storage
noun
1.
a computer system in which the size of the memory is effectively increased by automatically transferring sections of a program from a large capacity backing store, such as a disk, into the smaller core memory as they are required