The Collaborative International Dictionary of English v.0.48:

Acanthus \A*can"thus\, n.; pl. E. Acanthuses, L. Acanthi.
   [L., from Gr. ?. Cf. Acantha.]
   [1913 Webster]
   1. (Bot.) A genus of herbaceous prickly plants, found in the
      south of Europe, Asia Minor, and India; bear's-breech.
      [1913 Webster]

   2. (Arch.) An ornament resembling the foliage or leaves of
      the acanthus (Acanthus spinosus); -- used in the
      capitals of the Corinthian and Composite orders.
      [1913 Webster]

WordNet (r) 3.0 (2006):

acanthus
    n 1: any plant of the genus Acanthus having large spiny leaves
         and spikes or white or purplish flowers; native to
         Mediterranean region but widely cultivated