Swapping
A process can be swapped temporarily out of memory to a backing store and then brought back into memory for continued execution.
Backing store - fast disk large enough to accommodate copies of all memory images for all users.
Major part of swap time is transfer time, which is proportional to the amount of memory swapped.
Roll out, roll in - swapping used for priority-based medium term scheduling (lower priority process is swapped out so higher priority process can be loaded).