Virtual Memory
In plain terms
An abstraction that gives each program its own large, contiguous memory space.
Going deeper
The OS maps virtual addresses to physical RAM (and disk when needed), isolating programs and letting them use more memory than physically exists. Paging swaps data between RAM and disk.
Under the hood
Page tables map virtual to physical pages; the MMU translates addresses, and excessive paging to disk (thrashing) destroys performance.
Examples
Swap space, memory-mapped files, copy-on-write fork, page faults.
Think of it like...
Like giving every employee their own clean desk layout, while the office secretly shuffles papers to and from storage to fit everyone in limited space.