What type of nut is thin and installed between the main nut to prevent the main nut from turning?

The correct response is the jam nut. A jam nut is specifically designed for use with a main nut to secure it in place and prevent it from loosening due to vibrations or other forces. It is thinner than the main nut, allowing it to fit snugly in place against the primary nut. By being installed next to the main nut, the jam nut effectively "jams" the threads, creating friction and resistance against any unwinding motion that might occur under operational conditions.

In contrast, while a lock nut is often intended to perform a similar function of preventing loosening, it typically features a more complex design that can include a nylon insert or slotted structure, which is not specifically the thin profile associated with jam nuts. Hex nuts and wing nuts serve different purposes, with hex nuts commonly used in general applications and wing nuts designed for hand-tightening. The unique characteristic of the jam nut being thin and its strategic placement alongside another nut is what distinguishes it in this context.