1a plant of the milkweed family, the follicles of which suggest a swallow with outstretched wings, often becoming a weed.
Several species in the family Asclepiadaceae, in particular the European black swallow-wort (Cynanchum (or Vincetoxicum) nigrum), and the American Asclepias curassavica
2British the greater celandine, formerly believed to be used by swallows to restore their sight.