The Collaborative International Dictionary of English v.0.48:

Betaine \Be"ta*ine\, n. [From beta, generic name of the beet.]
   (Chem.)
   A nitrogenous base, C5H11NO2, produced artificially, and
   also occurring naturally in beet-root molasses and its
   residues, from which it is extracted as a white crystalline
   substance; -- called also lycine and oxyneurine. It has a
   sweetish taste.
   [1913 Webster]

WordNet (r) 3.0 (2006):

betaine
    n 1: a sweet tasting alkaloid that occurs in sugar beets