Source : WordNet®

RAM disk
     n : (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"

Source : Free On-Line Dictionary of Computing

RAM 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)