jackstay

Short definition from Wiktionary

Noun

1. 1
   A stay (rope, bar or batten), running along a ship's yard, to which is attached the head of a square sail.
2. 2
   A cable between two ships or from a ship to a fixed point which can be used to support a load during transfer of personnel or materiel along the cable.
3. 3
   A line (rope, webbing or cable), attached to a boat at the ends, to which a safety harness can be clipped to restrain falling in rough conditions and to prevent falling overboard.
4. 4
   A line fixed at both ends, which may be used to guide a load or a diver along the route of the line. Uses include guidance to and from the underwater work site, and as a means of controlling an underwater search.