a metal sleeve with opposite internal threads at each end for the threaded ends of two rods or for ringbolts, forming a coupling that can be turned to tighten or loosen the rods or wires attached to the ringbolts
See turnbuckle in American Heritage Dictionary 4
(tûrnˈbŭkˌəl)
noun
A metal coupling device consisting of an oblong piece internally threaded at both ends into which the corresponding sections of two threaded rods are screwed in order to form a unit that can be adjusted for tension or length.