The Collaborative International Dictionary of English v.0.48:

Big Board \Big Board\ (b[i^]g" b[^o]rd`), prop. n. (Finance)
   The New York Stock Exchange; -- a nickname often used in
   financial reporting.
   [PJC]

WordNet (r) 3.0 (2006):

big board
    n 1: a stock exchange in New York [syn: New York Stock
         Exchange, N. Y. Stock Exchange, NYSE, big board]
    2: the large display board at the New York Stock Exchange that
       reports on stocks traded on the exchange