Definition of "milkweed"

Any of several plants that have a milky sap and have pods that split to release seeds with silky tufts.

1. Principally of the genus Asclepias in order Gentianales, family Apocynaceae  examples

2. Also of the genus Calotropis in order Gentianales, family Apocynaceae  examples

3. Euphorbia peplus and Euphorbia heterophylla, in order Malpighiales.  examples

A monarch butterfly (Danaus spp).  examples