The Collaborative International Dictionary of English v.0.48:

Horehound \Hore"hound`\, n. [OE. horehune, AS. h[=a]rhune;
   h[=a]r hoar, gray + hune horehound; cf. L. cunila a species
   of organum, Gr. ?, Skr. kn?y to smell.] (Bot.)
   1. A plant of the genus Marrubium (Marrubium vulgare),
      which has a bitter taste, and is a weak tonic, used as a
      household remedy for colds, coughing, etc. [Written also
      hoarhound.]
      [1913 Webster]

   2. A lozenge or tablet, usually sweetened, containing extract
      of horehound, used as a remedy for a cough or a sore
      throat.
      [PJC]

   Fetid horehound, or Black horehound, a disagreeable plant
      resembling horehound (Ballota nigra).

   Water horehound, a species of the genus Lycopus,
      resembling mint, but not aromatic.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Marrubium vulgare
    n 1: European aromatic herb with hairy leaves and numerous white
         flowers in axillary cymes; leaves yield a bitter extract
         use medicinally and as flavoring [syn: common horehound,
         white horehound, Marrubium vulgare]