In Fedran, reactionals are a major classification of the Crystal Spheres. They describe the fundamental reactions between various elemental magic and contain concepts such as fire, lightning, and acid.
The idea of splitting elemental and reactional magic came from a single place, Rich Burlew's Order of the Stick. In specific comic #423. When I read that, the idea went straight to long term memory and was pulled into even the first stories I wrote in the world.