A piece of rigging hardware specifically for adjusting the length or tension of a line. The appropriateness of their use with dynamic loads is debated. Once a standard part of all counterweight rigging systems' linesets to level battens, many companies today will no longer supply them for that purpose.
Further subdivided by type of attachment. The above, from top to bottom are: hook-hook, jaw-jaw, and hook-eye. Hook type have no place in stage rigging. As to the other two, see this thread for discussion: https://www.controlbooth.com/threads/turnbuckle-style.40175/ .