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

shared memory

   1. Memory in a parallel computer, usually RAM, which can
   be accessed by more than one processor, usually via a shared
   bus or network.

   It usually takes longer for a processor to access shared
   memory than to access its own private memory because of
   contention for the processor-to-memory connections and because
   of other overheads associated with ensuring synchronised
   access.  Computers using shared memory usually have some kind
   of local cache on each processor to reduce the number of
   accesses to shared memory.  This requires a cache
   consistency protocol to ensure that one processor's cached
   copy of a shared memory location is invalidated when another
   processor writes to that location.

   The alternative to shared memory is message passing where
   all memory is private to some particular processor and
   processors communicate by sending messages down special
   links.  This is usually slower than shared memory but it
   avoids the problems of contention for memory and can be
   implemented more cheaply.

   2. Memory which can be access by more than one process in a
   multitasking operating system with memory protection.
   Some Unix variants, e.g. SunOS provide this kind of shared
   memory.

   Unix manual pages: shmop(2), shmctl(2), shmget(2).

   (1994-10-20)