virtual disk

from The Free On-line Dictionary of Computing (8 July 2008)

RAM disk
RAM drive
virtual disk

   <operating system, storage> A memory-resident program which
   mimics a {hard disk} drive.  It uses part of computer's {RAM}
   to store data which can be accessed as files.  Unlike a real
   disk drive, this drive forgets all stored data when the
   computer is turned off.

   (1995-03-14)