Virtual memory
From Mac Guides
Virtual memory is a memory management technique that allows software to use memory address space not necessarily tied to the computer's physical memory (eg RAM). Typically the hard drive is used instead.
Virtual memory was first introduced in Mac OS with System 7 in 1991. Until Mac OS X, it was optional, however it is now used extensively by the system. It is not unusual to have up to ten gigabytes of virtual memory, depending on the applications you use and how much memory you have installed. A large portion of what Mac OS X calls virtual memory, however, is only mapped into memory, i.e. it wasn't swapped out of RAM, but rather is used as a cache.
Although virtual memory is useful in that it effectively increases the capacity of your memory, accessing the hard drive is much slower than physical memory. For this reason, it is often advisable to install additional RAM if virtual memory is used extensively on your computer.
