any of a genus (Penicillium) of imperfect fungi growing as green mold on stale bread, ripening cheese, decaying fruit, etc.: penicillin is derived from some species
See penicillium in American Heritage Dictionary 4
(pĕnˌĭ-sĭlˈē-əm)
nounpl.pen·i·cil·li·ums or pen·i·cil·li·a(-sĭlˈē-ə)
Any of various characteristically bluish-green fungi of the genus Penicillium that grow as molds on decaying fruits and ripening cheese and are used in the production of penicillin and in making cheese.