The Collaborative International Dictionary of English v.0.48:

Rosebay \Rose"bay`\, n. (Bot.)
      (a) the oleander. [Obs.]
      (b) Any shrub of the genus Rhododendron. [U.S.]
      (c) An herb (Epilobium spicatum) with showy purple
          flowers, common in Europe and North America; -- called
          also great willow herb.
          [1913 Webster]