A RAM disk or RAMdrive is a segment of active computer memory, RAM, which is being used as secondary storage, typically a role for a disk drive. Access times are greatly improved, because RAM disks are approximately a thousand times faster than hard disk drives. However, durability of data through power loss is completely absent, because they are made of normal RAM. As a result, RAM disks lose their contents once the computer is turned off. RAM disks are great places to store temporary data or to hold uncompressed programs for short periods.

A proper disk cache in the operating system will usually obviate the performance motivation for a RAM disk; a disk cache fulfills a similar role (fast access to data that is notionally stored on a disk) without the various penalties (data loss in the event of power loss, static partitioning, etc.). RAM disks are, however, indispensable in situations in which a physical disk is not available, or where access to, or changing a physical disk is not desirable (such as in the case of LiveCDs).

Another way to use RAM to store files is the temporary filesystem. The difference between temporary filesystem and a RAM disk is that the RAM disk (/dev/ram0 etc.) is fixed-sized and acts like a disk partition, whereas the temporary filesystem (/dev/shm; in Source Mage GNU/Linux also /tmp) grows and shrinks to fit the files put on it.