NOUN (3)

1. used especially in treating bruises;

2. any of various rhizomatous usually perennial plants of the genus Arnica;

3. an ointment used in treating bruises;

The Collaborative International Dictionary of English v.0.48:

Arnica \Ar"ni*ca\, n. [Prob. a corruption of ptarmica.] (Bot.)
   A genus of plants; also, the most important species (Arnica
   montana), native of the mountains of Europe, used in
   medicine as a narcotic and stimulant.
   [1913 Webster]

   Note: The tincture of arnica is applied externally as a
         remedy for bruises, sprains, etc.
         [1913 Webster]

WordNet (r) 3.0 (2006):

arnica
    n 1: used especially in treating bruises
    2: any of various rhizomatous usually perennial plants of the
       genus Arnica
    3: an ointment used in treating bruises