Source : Webster's Revised Unabridged Dictionary (1913)

Oidium \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.