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]

The Free On-line Dictionary of Computing (30 December 2018):

main memory
main store
primary storage

    The storage device used by a
   computer to hold the currently executing program and its
   working data.  A modern computer's main memory is built from
   random-access memory integrated circuits.  In the old days
   ferrite core memory was one popular form of main memory,
   leading to the use of the term "core" for main memory.

   Computers have several other sorts of memory, distinguished by
   their access time, storage capicity, cost, and the typical
   lifetime or rate of change of the data they hold.  Registers
   in the CPU are fast, few, expensive and typically change
   every few machine instructions.  Other kinds are cache,
   PROM, magnetic disk (which may be used for virtual
   memory) and magnetic tape.

   (1996-11-04)