an auxiliary sail, often triangular, set to catch the wind, as between the masts of a yawl (mizzen staysail), aft of a spinnaker (spinnaker staysail), etc
staysail in American English
(ˈsteɪˌseɪl; Nautical ˈsteɪsəl)
noun
a triangular, fore-and-aft-rigged sail fastened on a stay