The Collaborative International Dictionary of English v.0.48:

Asclepias \As*cle"pi*as\, n. [L., fr. Gr. ?, named from
   Asclepios or Aesculapius.] (Bot.)
   A genus of plants including the milkweed, swallowwort, and
   some other species having medicinal properties.
   [1913 Webster]

   Asclepias butterfly (Zool.), a large, handsome, red and
      black butterfly (Danais Archippus), found in both
      hemispheres. It feeds on plants of the genus Asclepias.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Asclepias
    n 1: genus of chiefly North American perennial herbs: silkweed;
         milkweed [syn: Asclepias, genus Asclepias]