The Collaborative International Dictionary of English v.0.48:

Baluster \Bal"us*ter\, n. [F. balustre, It. balaustro, fr. L.
   balaustium the flower of the wild pomegranate, fr. Gr.
   balay`stion; -- so named from the similarity of form.]
   (Arch.)
   A small column or pilaster, used as a support to the rail of
   an open parapet, to guard the side of a staircase, or the
   front of a gallery. See Balustrade. [Corrupted into
   banister.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

baluster
    n 1: one of a number of closely spaced supports for a railing