On a sailing vessel, a forestay, sometimes just called a stay, is a piece of standing rigging which keeps a mast from falling backwards. It is attached either at the very top of the mast, or in fractional rigs between about 1/8 and 1/4 from the top of the mast. The other end of the forestay is attached to the bow of the boat.
Often a sail is attached to the forestay. This sail may be a jib or a genoa. In a cutter rig, the jib or jibs are flown from stays in front of the forestay, perhaps going from the masthead to a bowsprit. The sail on the forestay is then referred to as the staysail or stays’l.
A forestay might be made from stainless steel wire on a modern yacht, solid stainless steel rod, carbon rod, or ultra high molecular weight polyethylene (such as Spectra or Dyneema) on a high-performance racing boat, and galvanised wire or natural fibers on an older cutter or square-rigged ship.