The Collaborative International Dictionary of English v.0.48:

main memory \main memory\ n. (Computers)
   The memory in a computer that holds programs and data for
   rapid access during execution of a program; it usually hold
   the largest quantity of rapid-access storage in a computer;
   -- also called RAM (random access memory. It is
   contrasted to ROM, disk data storage, cache,
   registers and other forms of data storage.
   [PJC]

WordNet (r) 3.0 (2006):

random access memory
    n 1: the most common computer memory which can be used by
         programs to perform necessary tasks while the computer is
         on; an integrated circuit memory chip allows information to
         be stored or accessed in any order and all storage
         locations are equally accessible [syn: random-access
         memory, random access memory, random memory, RAM,
         read/write memory]