Source : Webster's Revised Unabridged Dictionary (1913)

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      is now superseded by indigo, but is somewhat used with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.

Source : WordNet®

woad
     n 1: a blue dyestuff obtained from the woad plant
     2: any of several herbs of the genus Isatis