Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Source : WordNet®

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