O-īd'/i-ūm, n.

n. [Gr. οἰδεῖν, to swell up.] (Bot.) A parasitic fungus consisting of minute interlacing filaments, that cover the surface with a white down. It is probably the mycelium of various fungi. One species (O. Tuckeri) is very injurious to the grape-vine, and is known as the vine-mildew.