Jackstay

1. A rope, bar, or batten running along a ship's yard, to which is attached the head of a square sail.
2. A stay for racing or cruising vessels used to steady the mast against the strain of the gaff.
3. A cable between two ships or from a ship to a fixed point that supports a load during transfer of personnel or materiel along the cable.
4. On a yacht, a deck lifeline of rope or (preferably) flat tape may be called a jackstay, though this is a misnomer as a jackstay is a stay rather than a line.