thixotropic agent (plastic resin component). A material, such as microballoons, added to a resin to give it body and increase its workability.
thorium. A heavy, silvery-white, malleable, metallic chemical element. Thorium’s symbol is Th, its atomic number is 90, and its atomic weight is 232. Thorium is slightly radioactive and is used as an alloy with tungsten to make filaments for incandescent electric lights. It is also a potential source of nuclear energy.
thread (machine fastener component). Helical grooves cut onto the surface of a bolt or screw. Mating threads are cut into the inside of a nut. The threads inside the nut match the threads on the outside of the bolt, and the nut is turned down over the threads of the bolt to produce a clamping action that holds parts of a machine together. A threaded bolt is a form of inclined plane.
thread (textiles). Long filaments made by twisting together short lengths of natural fibers, such as cotton or linen, to form a continuous strand. Threads of synthetic materials are made of a single continuous filament. These are called monofilament threads.
thread pitch (threaded fastener specification). The linear distance from the apex (peak) of one thread to the similar position on the next thread on a threaded fastener.

three-axis automatic pilot. An automatic flight control system for an aircraft that controls the movement of the aircraft about all three of its axes.