What defines any machine that can derive support in the atmosphere from air reactions?

The term that encompasses any machine capable of deriving support in the atmosphere from air reactions is "aircraft." This definition includes a wide variety of flying machines, such as airplanes, helicopters, and even more unconventional designs like ornithopters. All these machines utilize aerodynamic principles to generate lift and navigate through the air.

While airplanes and helicopters are specific types of aircraft, the term "aircraft" serves as the broader category that includes all flying machines. An ornithopter, which mimics the flapping motion of birds, is another example of an aircraft but also falls under the specific category rather than serving as the all-encompassing definition. Thus, the classification as "aircraft" accurately describes the general concept of machines that derive support from air reactions, making it the most appropriate choice in this context.