A jackstay is a secured line, wire, or rod (often webbing) running along a boat's deck, to which crew clip their safety harnesses to prevent falling overboard. It acts as a safety lifeline during heavy weather, allowing movement between the bow and stern. It also refers to a spar rod for attaching sails.

Key Aspects of a Jackstay:
Safety Use (Jackline): Primarily, it is a strap or wire running from bow to stern on both sides of the deck, allowing crew to remain clipped in with a harness tether while moving around.

Rigging Use: A metal rod, wire rope, or wooden batten running along a yard or boom, used to bend (fasten) the edge of a sail.

Material: Usually made of flat webbing (which doesn't roll underfoot) or vinyl-coated wire.

Note: While "jackstay" is commonly used for safety lines, they are often more specifically termed "jacklines," though the terms are frequently used interchangeably.