Source : Free On-Line Dictionary of Computing

presence detect
     
         A means of identifying a memory chip to the memory
        controller logic.  The original scheme, called Parallel
        Presence Detect (PPD) used a separate pin for each bit of
        information.  As the number of pins can not be very large this
        only allowed to identify the density and the speed of the
        chips.  To allow for additional information the Serial
        Presence Detect (SPD) scheme was introduced which uses a
        serial EEPROM to store the presence detect information and
        requires only two pins (one for enabling it and one for data).
     
        (1998-03-07)