The Collaborative International Dictionary of English v.0.48:

Mullein \Mul"lein\, n. [OE. moleyn, AS. molegn.] (Bot.)
   Any plant of the genus Verbascum. They are tall herbs
   having coarse leaves, and large flowers in dense spikes. The
   common species, with densely woolly leaves, is Verbascum
   Thapsus.
   [1913 Webster]

   Moth mullein. See under Moth.

   Mullein foxglove, an American herb (Seymeria macrophylla)
      with coarse leaves and yellow tubular flowers with a
      spreading border.

   Petty mullein, the cowslip. --Dr. Prior.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mullein
    n 1: any of various plants of the genus Verbascum having large
         usually woolly leaves and terminal spikes of yellow or
         white or purplish flowers [syn: mullein, flannel leaf,
         velvet plant]