jackstay (plural jackstays)
- (nautical) A stay (rope, bar or batten), running along a ship's yard, to which is attached the head of a square sail.
- (nautical) 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.
- (nautical) 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.
- (underwater diving) 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.