Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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: {silkweed}]
     2: annual Eurasian sow thistle with soft spiny leaves and rayed
        yellow flower heads [syn: {Sonchus oleraceus}]