The Collaborative International Dictionary of English v.0.48:

Sarsaparilla \Sar`sa*pa*ril"la\, n. [Sp. zarzaparrilla; zarza a
   bramble (perhaps fr. Bisc. zartzia) + parra a vine, or
   Parillo, a physician said to have discovered it.] (Bot.)
   (a) Any plant of several tropical American species of
       Smilax.
   (b) The bitter mucilaginous roots of such plants, used in
       medicine and in sirups for soda, etc.
       [1913 Webster]

   Note: The name is also applied to many other plants and their
         roots, especially to the Aralia nudicaulis, the wild
         sarsaparilla of the United States.
         [1913 Webster]

WordNet (r) 3.0 (2006):

sarsaparilla
    n 1: any of various prickly climbing plants of the tropical
         American genus Smilax having aromatic roots and heart-
         shaped leaves
    2: carbonated drink flavored with an extract from sarsaparilla
       root or with birch oil and sassafras