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

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "gasket":
   air hole, armhole, blowhole, bullet-hole, bunghole, cringle,
   deadeye, eye, eyelet, gland, grommet, guide, keyhole, knothole,
   loop, loophole, manhole, mousehole, packing, padding, peephole,
   pigeonhole, pinhole, placket, placket hole, porthole, punch-hole,
   spiracle, stopping, stuffing, tampon, tap, vent, venthole,
   wadding