[oid.i.um] n, pl -ia [NL, fr. o- + -idium] (1857) 1 a: any of a genus (Oidium of the family Moniliaceae) of imperfect fungi many of which are now considered to be conidial stages of various powdery mildews b: one of the small conidia borne in chains by various fungi (as an oidium)--called also arthrospore
2: a powdery mildew caused by an oidium esp. in the grape