The Collaborative International Dictionary of English v.0.48:

Silk \Silk\, n. [OE. silk, selk, AS. seolc, seoloc; akin to
   Icel. silki, SW. & Dan. silke; prob. through Slavic from an
   Oriental source; cf. Lith. szilkai, Russ. shelk', and also L.
   sericum Seric stuff, silk. Cf. Sericeous. Serge a woolen
   stuff.]
   1. The fine, soft thread produced by various species of
      caterpillars in forming the cocoons within which the worm
      is inclosed during the pupa state, especially that
      produced by the larvae of Bombyx mori.
      [1913 Webster]

   2. Hence, thread spun, or cloth woven, from the above-named
      material.
      [1913 Webster]

   3. That which resembles silk, as the filiform styles of the
      female flower of maize.
      [1913 Webster]

   Raw silk, silk as it is wound off from the cocoons, and
      before it is manufactured.

   Silk cotton, a cottony substance enveloping the seeds of
      the silk-cotton tree.

   Silk-cotton tree (Bot.), a name for several tropical trees
      of the genera Bombax and Eriodendron, and belonging to
      the order Bombaceae. The trees grow to an immense size,
      and have their seeds enveloped in a cottony substance,
      which is used for stuffing cushions, but can not be spun.


   Silk flower. (Bot.)
      (a) The silk tree.
      (b) A similar tree (Calliandra trinervia) of Peru.

   Silk fowl (Zool.), a breed of domestic fowls having silky
      plumage.

   Silk gland (Zool.), a gland which secretes the material of
      silk, as in spider or a silkworm; a sericterium.

   Silk gown, the distinctive robe of a barrister who has been
      appointed king's or queen's counsel; hence, the counsel
      himself. Such a one has precedence over mere barristers,
      who wear stuff gowns. [Eng.]

   Silk grass (Bot.), a kind of grass (Stipa comata) of the
      Western United States, which has very long silky awns. The
      name is also sometimes given to various species of the
      genera Aqave and Yucca.

   Silk moth (Zool.), the adult moth of any silkworm. See
      Silkworm.

   Silk shag, a coarse, rough-woven silk, like plush, but with
      a stiffer nap.

   Silk spider (Zool.), a large spider (Nephila plumipes),
      native of the Southern United States, remarkable for the
      large quantity of strong silk it produces and for the
      great disparity in the sizes of the sexes.

   Silk thrower, Silk throwster, one who twists or spins
      silk, and prepares it for weaving. --Brande & C.

   Silk tree (Bot.), an Asiatic leguminous tree (Albizzia
      Julibrissin) with finely bipinnate leaves, and large flat
      pods; -- so called because of the abundant long silky
      stamens of its blossoms. Also called silk flower.

   Silk vessel. (Zool.) Same as Silk gland, above.

   Virginia silk (Bot.), a climbing plant (Periploca
      Gr[ae]ca) of the Milkweed family, having a silky tuft on
      the seeds. It is native in Southern Europe.
      [1913 Webster]

WordNet (r) 3.0 (2006):

silk cotton
    n 1: a plant fiber from the kapok tree; used for stuffing and
         insulation [syn: kapok, silk cotton, vegetable silk]