The Collaborative International Dictionary of English v.0.48:

lateen sail \lateen sail\ n. [F. voile latine a sail in the
   shape of a right-angled triangle; cf. It. & Sp. vela latina;
   properly Latin sail. See Latin.] (Naut.)
   A triangular sail, extended by a long yard, which is slung at
   about one fourth of its length from the lower end, to a low
   mast, this end being brought down at the tack, while the
   other end is elevated at an angle or about forty-five
   degrees; -- used in small boats, feluccas, xebecs, etc.,
   especially in the Mediterranean and adjacent waters. Some
   lateen sails have also a boom on the lower side.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lateen sail
    n 1: a triangular fore-and-aft sail used especially in the
         Mediterranean [syn: lateen, lateen sail]