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.

The Collaborative International Dictionary of English v.0.48:

bannister \bannister\ n.
   same as banister.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

Bannister
    n 1: English runner who in 1954 became the first person to run a
         mile in less than four minutes (born in 1929) [syn:
         Bannister, Roger Bannister, Sir Roger Gilbert
         Bannister]
    2: a railing at the side of a staircase or balcony to prevent
       people from falling [syn: bannister, banister,
       balustrade, balusters, handrail]