Refers to RAM that has been configured to simulate a disk drive. You can access files on a RAM disk as you would access files on a real disk. RAM disks, however, are approximately a thousand times faster than hard disk drives. They are particularly useful, therefore, for applications that require frequent disk accesses.
Because they are made of normal RAM, RAM disks lose their contents once the computer is turned off. To use a RAM disk, therefore, you need to copy files from a real hard disk at the beginning of the session and then copy the files back to the hard disk before you turn the computer off. Note that if there is a power failure, you will lose whatever data is on the RAM disk. (Some RAM disks come with a battery backup to make them more stable.)

A RAM disk is also called a virtual disk or a RAM drive.