The Collaborative International Dictionary of English v.0.48:

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.]
   [1913 Webster]
   1. (Bot.) An herbaceous cruciferous plant (Isatis
      tinctoria) of the family Cruciferae (syn.
      Brassicaceae). It was formerly cultivated for the blue
      coloring matter derived from its leaves. See isatin.
      [1913 Webster]

   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      It is now superseded by indigo, but is somewhat used with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.
      [1913 Webster]

            Their bodies . . . painted with woad in sundry
            figures.                              --Milton.
      [1913 Webster]

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


   Woad mill, a mill grinding and preparing woad.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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