The Collaborative International Dictionary of English v.0.48:

Milkweed \Milk"weed`\, n. (Bot.)
   Any plant of the genera Asclepias and Acerates, abounding
   in a milky juice, and having its seed attached to a long
   silky down; silkweed. The name is also applied to several
   other plants with a milky juice, as to several kinds of
   spurge. Its leaves are a favorite food source for the larvae
   of the monarch butterfly.
   [1913 Webster]

WordNet (r) 3.0 (2006):

milkweed
    n 1: any of numerous plants of the genus Asclepias having milky
         juice and pods that split open releasing seeds with downy
         tufts [syn: milkweed, silkweed]
    2: annual Eurasian sow thistle with soft spiny leaves and rayed
       yellow flower heads [syn: milkweed, Sonchus oleraceus]