Autodesk's HDI is short for Hierarchical Object-Oriented Picture System Extended Immediate-mode Drawing Interface Device Interface.
It turns out that HDI is composed of two other acronyms:
-- HDI is short for HEIDI Device Interface.
-- HEIDI is short for HOOPS Extended Immediate-mode Drawing Interface.
-- and HOOPS is short for Hierarchical Object-Oriented Picture System.
So that's how the three-letter acronym HDI becomes an abbreviation for 12 words.