The slender recoil spring which regulates the motion of the balance in a timepiece.

The delicate, spiral-wound spring which governs the oscillation of the balance wheel and provides the fundamental timekeeping mechanism of the watch. Hairsprings can be "flat" or "overcoiled" in design. The shape of the overcoil or terminal curve of the hairpspring can have a significant effect on the positional accuracy of the watch. Hairsprings can be made of blue steel or other more exotic metal alloys such as "Invar" or "Elinvar" which are formulated to resist the adverse effects of temperature or magnetism.

a fine spiral spring that regulates the movement of the balance wheel in a timepiece