\O*["i]d"i*um\, n. [NL., dim. fr. Gr. w,'o`n egg.] (Bot.)
A genus of minute fungi which form a floccose mass of
filaments on decaying fruit, etc. Many forms once referred to
this genus are now believed to be temporary conditions of
fungi of other genera, among them the vine mildew ({O["i]dium
Tuckeri}), which has caused much injury to grapes.