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

Serial Peripheral Interface

    (SPI) A serial interface in which
   a master device supplies clock pulses to exchanges data
   serially with a slave over two data wires (Master-Slave and
   Slave-Master).  This term probably originated with Motorola in
   about 1979 with their first all-in-one microcontroller.

   (2003-07-13)