(1):

(n.) An herbaceous cruciferous plant (Isatis tinctoria). It was formerly cultivated for the blue coloring matter derived from its leaves.

(2):

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 It is now superseded by indigo, but is somewhat used with indigo as a ferment in dyeing.