<storage> (RAM) (Previously "direct-access memory"). A data storage device for which the order of access to different locations does not affect the speed of access. This is in contrast to, say, a magnetic disk, magnetic tape or a mercury delay line where it is very much quicker to access data sequentially because accessing a non-sequential location requires physical movement of the storage medium rather than just electronic switching.
In the 1970s magnetic core memory was used and some old-timers still call RAM "core". The most common form of RAM in use today is semiconductor integrated circuits, which can be either static random-access memory (SRAM) or dynamic random-access memory (DRAM).
The term "RAM" has gained the additional meaning of read-write. Most kinds of semiconductor read-only memory (ROM) are actually "random access" in the above sense but are never referred to as RAM. Furthermore, memory referred to as RAM can usually be read and written equally quickly (approximately), in contrast to the various kinds of programmable read-only memory. Finally, RAM is usually volatile though non-volatile random-access memory is also used.
The humorous expansion "Rarely Adequate Memory" refers to the fact that programs and data always seem to expand to fill the memory available.