The Collaborative International Dictionary of English v.0.48:

Marrubium \Mar*ru"bi*um\, n. [L.] (Bot.)
   A genus of bitter aromatic plants, sometimes used in
   medicine; hoarhound.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Marrubium
    n 1: Old World aromatic herbs: horehound [syn: Marrubium,
         genus Marrubium]