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Webster 1913 Edition

Noun.

(Chem.)

A reddish brown amorphous dyestuff,

C7H7NO3

, obtained from orcin, and forming the essential coloring matter of cudbear and archil. It is closely related to litmus.

orcein ‎(usually uncountable, plural orceins)

(chemistry) a dye, related to litmus, that is extracted from the lichen Rocella tinctoria; used as a microscopic stain and as a food colouring