Definitions for hairspringˈhɛərˌsprɪŋ
Random House Webster's College Dictionary
a fine, usu. spiral, spring used for oscillating the balance of a timepiece.
Origin of hairspring:
a fine spiral spring that regulates the movement of the balance wheel in a timepiece
A spring, made of a coil of fine wire, that is used to regulate the movement of a balance wheel in a watch.
the slender recoil spring which regulates the motion of the balance in a timepiece