The Collaborative International Dictionary of English v.0.48:

Gasket \Gas"ket\, n. [Cf. F. garcette, It. gaschetta, Sp. cajeta
   caburn, garceta reef point.]
   1. (Naut.) A line or band used to lash a furled sail
      securely. Sea gaskets are common lines; harbor gaskets
      are plaited and decorated lines or bands. Called also
      casket.
      [1913 Webster]

   2. (Mech.)
      (a) The plaited hemp used for packing a piston, as of the
          steam engine and its pumps.
      (b) Any ring or washer of made of a compressible material,
          used to make joints impermeable to fluids.
          [1913 Webster]

WordNet (r) 3.0 (2006):

gasket
    n 1: seal consisting of a ring for packing pistons or sealing a
         pipe joint