The Collaborative International Dictionary of English v.0.48:

Buenos Aires \Buenos Aires\ prop. n. (Geography)
   The capital city of Argentina. Population (2000) = 3,000,000.
   [PJC]

WordNet (r) 3.0 (2006):

Buenos Aires
    n 1: capital and largest city of Argentina; located in eastern
         Argentina near Uruguay; Argentina's chief port and
         industrial and cultural center [syn: Buenos Aires,
         capital of Argentina]