‖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 (Oïdium Tuckeri), which has caused much injury to grapes. [ 1913 Webster ]