mammillaria UK /ˌmæmɪˈlɛəɹi.ə/US /ˌmæməˈlɛɹi.ə/ noun1 •Any member of the cactus genus Mammillaria.For some years back collections of mammillarias, from whatever parts of the world they came, were grown in our hothouses, and many of them are still kept there. — 1841 formsmammillarias