The Collaborative International Dictionary of English v.0.48:

Hawkweed \Hawk"weed`\ (-w[=e]d`), n. (Bot.)
   (a) A plant of the genus Hieracium; -- so called from the
       ancient belief that birds of prey used its juice to
       strengthen their vision.
   (b) A plant of the genus Senecio (Senecio hieracifolius).
       --Loudon.
       [1913 Webster]

WordNet (r) 3.0 (2006):

hawkweed
    n 1: any of various plants of the genus Pilosella
    2: any of numerous often hairy plants of the genus Hieracium
       having yellow or orange flowers that resemble the dandelion