Penicillium (class Hyphomycetes) A form genus of fungi which form a well developed, septate mycelium and branched or unbranched conidiophores bearing phialids. Conidia appear as greyish-green or bluish-green powdery masses on white mycelium. Most species are saprotrophic and cosmopolitan in distribution. Many strains produce important antibiotics, including penicillins; many species form toxins. Some species are used in the manufacture of certain types of cheese.

Penicillium A genus of moulds; apart from the production of penicillin, several species are valuable in the ripening of cheeses.

Penicillium (pen-i-sil-iŭm) n. a genus of mouldlike fungi that commonly grow on decaying fruit, bread, or cheese. Some species are pathogenic to humans, causing diseases of the skin and respiratory tract. P. rubrum the major natural source of the antibiotic penicillin.