Source : Webster's Revised Unabridged Dictionary (1913)

Aaron's rod \Aar"on's rod`\ ([^a]r"[u^]nz r[o^]d`). [See Exodus
   vii. 9 and Numbers xvii. 8]
   1. (Arch.) A rod with one serpent twined around it, thus
      differing from the caduceus of Mercury, which has two.

   2. (Bot.) A plant with a tall flowering stem; esp. the great
      mullein, or hag-taper, and the golden-rod.

Source : WordNet®

Aaron's rod
     n : tall-stalked very woolly mullein with densely packed yellow
         flowers; ancient Greeks and Romans dipped the stalks in
         tallow for funeral torches [syn: {common mullein}, {great
         mullein}, {flannel mullein}, {woolly mullein}, {torch}, {Verbascum
         thapsus}]