The Free On-line Dictionary of Computing (19 January 2023):

presence detect
Parallel Presence Detect
PPD
Serial Presence Detect
SPD

    A means of identifying a memory chip to the
   memory controller logic.  The original scheme, Parallel
   Presence Detect (PPD) used a separate pin for each bit of
   information.  The limited number of pins available only gave
   the density and the speed of the chips.  To pass more
   information, the Serial Presence Detect (SPD) scheme was
   introduced, storing the information in serial EEPROM with
   one pin to enable it and one for data.

   (2012-06-25)