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]

The Collaborative International Dictionary of English v.0.48:

Banister \Ban"is*ter\, n. [A corruption of baluster.]
   1. A baluster.
      [1913 Webster]

   2. (sing. or pl.) The balustrade of a staircase. Formerly
      used in this sense mostly in the plural, now mostly in the
      singular. [Also spelled bannister.]
      [1913 Webster +PJC]

            He struggled to ascend the pulpit stairs, holding
            hard on the banisters.                --Sir W.
                                                  Scott.

WordNet (r) 3.0 (2006):

banister
    n 1: a railing at the side of a staircase or balcony to prevent
         people from falling [syn: bannister, banister,
         balustrade, balusters, handrail]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "banister":
   Samson post, baluster, balustrade, base, caryatid, colonnade,
   column, dado, die, doorjamb, doorpost, footstalk, gatepost,
   hitching post, jack, jamb, king post, milepost, mullion,
   newel-post, pedestal, pedicel, peduncle, pier, pilaster, pile,
   piling, pillar, plinth, pole, post, queen-post, rail, shaft,
   signpost, snubbing post, socle, staff, stalk, stanchion, stand,
   standard, stem, stile, subbase, surbase, trunk, upright