X-Plane Flight Simulator/Glossary

Instrument Flight Rules - regulations and procedures for flying aircraft where navigation and obstacle clearance is maintained with reference to aircraft instruments only and separation from other aircraft is provided by Air Traffic Control.
Software Development Kit - a set of development tools that allow a software engineer to create applications for a certain software package, software framework, hardware platform, computer system, video game console, operating system, or similar.
Visual Flight Rules - regulations under which a pilot may operate an aircraft in weather conditions sufficient to allow the pilot to control the aircraft's attitude, navigate, and maintain safe separation from obstacles such as terrain, buildings, and other aircraft by visual reference to the environment outside the cockpit.