lactariusEnglishOxford New American Dictionarylactarius /ˌlakˈterēəs, ˌlækˈtɛriəs / ▸ noun a large woodland mushroom with a concave cap, the flesh exuding a white or colored milky fluid when cut. Genus Lactarius, family Russulaceae, class Hymenomycetes: several species, including the edible orange milk lactarius (L. deliciosus).