Source : Webster's Revised Unabridged Dictionary (1913)

   2. (Chem.) A blue dyestuff obtained from several plants
      belonging to very different genera and orders; as, the
      woad, {Isatis tinctoria}, {Indigofera tinctoria}, {I.
      Anil}, {Nereum tinctorium}, etc. It is a dark blue earthy
      substance, tasteless and odorless, with a copper-violet
      luster when rubbed. Indigo does not exist in the plants as
      such, but is obtained by decomposition of the glycoside
      indican.

   Note: Commercial indigo contains the essential coloring
         principle indigo blue or indigotine, with several other
         dyes; as, indigo red, indigo brown, etc., and various
         impurities. Indigo is insoluble in ordinary reagents,
         with the exception of strong sulphuric acid.

   {Chinese indigo} (Bot.), {Isatis indigotica}, a kind of woad.
      

   {Wild indigo} (Bot.), the American herb {Baptisia tinctoria}
      which yields a poor quality of indigo, as do several other
      species of the same genus.

Isatis \I"sa*tis\ (?; 277), n. [L., a kind of plant, Gr. ?
   woad.] (Bot.)
   A genus of herbs, some species of which, especially the
   {Isatis tinctoria}, yield a blue dye similar to indigo; woad.

Woad \Woad\, n. [OE. wod, AS. w[=a]d; akin to D. weede, G. waid,
   OHG. weit, Dan. vaid, veid, Sw. veide, L. vitrum.] [Written
   also {wad}, and {wade}.]
   1. (Bot.) An herbaceous cruciferous plant ({Isatis
      tinctoria}). It was formerly cultivated for the blue
      coloring matter derived from its leaves.

   2. A blue dyestuff, or coloring matter, consisting of the
      powdered and fermented leaves of the Isatis tinctoria. It
      is now superseded by indigo, but is somewhat used with
      indigo as a ferment in dyeing.

            Their bodies . . . painted with woad in sundry
            figures.                              --Milton.

   {Wild woad} (Bot.), the weld ({Reseda luteola}). See {Weld}.
      

   {Woad mill}, a mill grinding and preparing woad.

Pastel \Pas"tel\, n. [F.; cf. It. pastello. Cf. {Pastil}.]
   1. A crayon made of a paste composed of a color ground with
      gum water. [Sometimes incorrectly written {pastil}.]
      ``Charming heads in pastel.'' --W. Black.

   2. (Bot.) A plant affording a blue dye; the woad ({Isatis
      tinctoria}); also, the dye itself.

Source : WordNet®

Isatis tinctoria
     n : European biennial formerly grown for the blue coloring
         matter yielded by its leaves [syn: {dyer's woad}]