WordNet (r) 3.0 (2006):

RAM disk
    n 1: (computer science) a virtual drive that is created by
         setting aside part of the random-access memory to use as if
         it were a group of sectors; "access to a RAM disk is very
         fast but the data it contains is lost when the system is
         turned off"

The Free On-line Dictionary of Computing (30 December 2018):

RAM disk
RAM drive
virtual disk

    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)