HDK
The HDK is a comprehensive set of C++ libraries. These are the same libraries that the Side Effects programmers use to develop the Houdini family of products. With the HDK, you can create plugins which customize different areas in the Houdini interface.
HDK: Houdini Development Kit
Houdini Development Kit の略称
C++ 向けの一番ネイティブな Houdini の API
Refs
HDKについて - Qiita