What is virtual memory, and why is it useful even when a system has enough RAM?

Virtual memory gives processes their own address spaces and maps virtual addresses to physical memory or other backing storage through operating-system and hardware mechanisms. It supports isolation, controlled sharing, memory-mapped files, and flexible allocation. Paging to disk is one possible use, not the definition of virtual memory. A page fault can be a normal first access or file-backed load rather than a sign of insufficient RAM. Address-space size, resident memory, and committed memory measure different things and should not be treated as interchangeable.