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

Single In-line Memory Module

    (SIMM) A small circuit board or substrate, typically
   about 10cm x 2cm, with RAM integrated circuits or die on
   one or both sides and a single row of pins along one long
   edge.  Several SIMMs are mounted with their substrates at
   right-angles to the main circuit board (the motherboard).
   This configuration allows greater packing density than direct
   mounting of, e.g. DIL (dual in-line) RAM packages on the
   motherboard.  In 1993 one SIMM typically held one or four
   megabytes, by early 1997 one could hold 8, 16, or 32 MB.

   (1997-01-05)