Turnbuckle
A component in an aircraft control system used to adjust cable tension. A turnbuckle consists of a brass tubular barrel with right-hand threads in one end and left-hand in the other end. Control cable terminals screw into the two ends of the barrel, and turning the barrel pulls the terminals together, shortening the cable.
Aviation Maintenance Technician Handbook — Airframe, p. G-37
Taught in AF-C Procedures, AF-C Concepts.