The Collaborative International Dictionary of English v.0.48:

Kaolin \Ka"o*lin\, Kaoline \Ka"o*line\, n. [Chin. kao-ling.]
   (Min.)
   A very pure white clay, ordinarily in the form of an
   impalpable powder, and used to form the paste of porcelain;
   China clay; porcelain clay. It is chiefly derived from the
   decomposition of common feldspar.
   [1913 Webster]

   Note: The name is now applied to all porcelain clays which
         endure the fire without discoloration.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Clay \Clay\ (kl[=a]), n. [AS. cl[=ae]g; akin to LG. klei, D.
   klei, and perh. to AS. cl[=a]m clay, L. glus, gluten glue,
   Gr. gloio`s glutinous substance, E. glue. Cf. Clog.]
   1. A soft earth, which is plastic, or may be molded with the
      hands, consisting of hydrous silicate of aluminium. It is
      the result of the wearing down and decomposition, in part,
      of rocks containing aluminous minerals, as granite. Lime,
      magnesia, oxide of iron, and other ingredients, are often
      present as impurities.
      [1913 Webster]

   2. (Poetry & Script.) Earth in general, as representing the
      elementary particles of the human body; hence, the human
      body as formed from such particles.
      [1913 Webster]

            I also am formed out of the clay.     --Job xxxiii.
                                                  6.
      [1913 Webster]

            The earth is covered thick with other clay,
            Which her own clay shall cover.       --Byron.
      [1913 Webster]

   Bowlder clay. See under Bowlder.

   Brick clay, the common clay, containing some iron, and
      therefore turning red when burned.

   Clay cold, cold as clay or earth; lifeless; inanimate.

   Clay ironstone, an ore of iron consisting of the oxide or
      carbonate of iron mixed with clay or sand.

   Clay marl, a whitish, smooth, chalky clay.

   Clay mill, a mill for mixing and tempering clay; a pug
      mill.

   Clay pit, a pit where clay is dug.

   Clay slate (Min.), argillaceous schist; argillite.

   Fatty clays, clays having a greasy feel; they are chemical
      compounds of water, silica, and aluminia, as halloysite,
      bole, etc.

   Fire clay, a variety of clay, entirely free from lime,
      iron, or an alkali, and therefore infusible, and used for
      fire brick.

   Porcelain clay, a very pure variety, formed directly from
      the decomposition of feldspar, and often called kaolin.


   Potter's clay, a tolerably pure kind, free from iron.
      [1913 Webster]

WordNet (r) 3.0 (2006):

kaolin
    n 1: a fine usually white clay formed by the weathering of
         aluminous minerals (as feldspar); used in ceramics and as
         an absorbent and as a filler (e.g., in paper) [syn: china
         clay, china stone, kaolin, kaoline, porcelain
         clay, terra alba]