milkweedPronunciation: /ˈmɪlkwiːd/Definition of milkweed noun 1 [mass noun] a herbaceous American plant with milky sap, some kinds of which attract butterflies or yield a variety of useful products. Genus Asclepias, family Asclepiadaceae 2 (also milkweed butterfly)another term for monarch (sense 2).milkweed in other Oxford dictionaries Definition of milkweed in the US English dictionary