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Found one definition

1.                 From The Free On-line Dictionary of Computing (19 January 2023):
                   
   
   
                   
                     
   Mask Read-Only Memory
   MROM
      <storage> (MROM) A kind of ROM in which the memory contents
      are determined by one of the masks used to manufacture the
      integrated circuit.  MROM can give high storage density
      (bits per millimeter squared) making it a cheap solution for
      high volume applications.
      [Other ROM types?]
      (1995-04-22)