The NASA Scientific and Technical Information (STI) Program plays a key role in keeping NASA a leader in aeronautical and space sciences. The STI maintains NASA's database of aeronautical and space science information while also reporting on the Agency's research and development. This impressive thesaurus contains a hierarchical listing of all authorized terms contained in NASA's STI database, along with definitions. Although the large size of this PDF file might make navigation difficult, the document is an extremely valuable reference tool for librarians and students of aeronautics and space science.