Final answer:
Virtual memory is space on the hard drive used when the RAM is insufficient, involving swap space to extend memory capacity. Cache and buffer provide different functions and do not serve as substitutes for RAM capacity.
Step-by-step explanation:
Space on the hard drive that is used for data which doesn't fit in RAM is known as virtual memory. When the physical RAM is insufficient to handle the tasks being executed on a computer, the operating system can create virtual memory by using free space on the hard drive to simulate additional RAM. This process involves a file or a dedicated space on the hard drive called a swap file or swap space, where data from RAM is temporarily written to and read from, thus extending the total amount of memory available for running programs and operations.
While cache is a smaller, faster memory space that stores copies of the data from the most frequently used main memory locations, and buffer is a temporary holding area for data being transferred from one place to another, neither serves the same purpose as virtual memory in the context of substituting RAM capacity.