'Woad' definitions:

Definition of 'woad'

noun

A blue dyestuff obtained from the woad plant

noun

Any of several herbs of the genus Isatis

From: GCIDE

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 It is now superseded by indigo, but is somewhat used with indigo as a ferment in dyeing.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]