The Collaborative International Dictionary of English v.0.48:

Niagara \Niagara\, Niagara Falls \Niagara Falls\prop. n.
   A large waterfall on the Niagara River near Buffalo
   remarkable for the volume and constancy of the water flowing
   over it. That part of the waterfall in Canada is the
   Horseshoe Falls; the U. S. portion is called the American
   Falls.
   [WordNet 1.5] Niagara

WordNet (r) 3.0 (2006):

Niagara Falls
    n 1: waterfall in Canada is the Horseshoe Falls; in the United
         States it is the American Falls [syn: Niagara, Niagara
         Falls]
    2: a city in western New York State at the falls of the Niagara
       river; tourist attraction and honeymoon resort