The term originally referred to what is now usually called virtual memory, which was introduced in 1962 by the Atlas system at the University of Manchester.
The persistent object concept was first introduced by Multics in the mid-1960s, in a project shared by MIT, General Electric and Bell Labs.
In order to process data, programs would use explicit code to read data from secondary storage into main memory, manipulate it in main memory, and then use more code to write it back out to secondary storage again.
VM systems break the main memory down into a series of fixed-sized segments, or "pages", and allocate them to programs on demand.
As programs used up the physical memory, and it eventually ran out, the VM system would examine the pages for data that was not currently needed, and write it out to secondary storage.
A single-level store changes this model by extending VM from handling just a paging file to a new concept where the "main memory" is the entire secondary storage system.
IBM's design of the single-level storage was originally conceived and pioneered by Frank Soltis and Glenn Henry in the late 1970s as a way to build a transitional implementation to computers with 100% solid state memory.
The thinking at the time was that disk drives would become obsolete, and would be replaced entirely with some form of solid state memory.
This has not come to be, however, because while solid state memory has become exponentially cheaper, disk drives have also become similarly cheaper; thus, the price ratio in favour of disk drives continues: very much higher capacities than solid state memory, very much slower to access, and much less expensive.