The answer: SOFTIMAGE Plug-in Description Language

The question: What does SPDL stand for?

Cover page from the original SPDL specification:

The origins of SPDL go back to at least 1997. The original SPDL parser was written with BISON and FLEX (GNU versions of yacc and lex), and at one point I think there where three different parsers, two based on BISON/FLEX, and one C++/COM (that was back in the early Twister/Sumatra/DS days). I remember going through the BISON/FLEX files in order to write up a SPDL syntax doc…fun times!

FUN FACT: There are 1,511 spdl files in the Softimage 2013 SP1 installation folder.