IIRC, NA Bob Perry calls it a "tie rod". This goes from cabin top to either a strong point on the cabin sole or to the spar itself inside the cabin. This is needed on keel-stepped spars to counteract the up-force from the halyards going thru sheaves at the base of the spar to the housetop winches / clutches.
Our boat has a shorter piece of rigging wire and a turn buckle going down a couple of feet to the spar on the inside of our cabin. Ours goes to an SS tang inserted thru the house top just behind the mast penetration where the SS base fitting is fastened to the house top for all the halyard sheaves.
Also, some boats have the fitting that those sheaves are attached to supported by a pin thru the spar itself, which accomplishes the same mission.