Buenos Aires UK /ˈbweɪ.nəs ˈɛə.ɹiːz/US /ˈbweɪ.nəs ˈɛɹ.iz/ noun2 •The capital city of Argentina.Of this particular letter he remarked only that it was in a curious envelope, that it was in a man's handwriting, that the postscript was Buenos Ayres, and the stamp of the Argentine Republic. — 1922 •The largest and most populous province in Argentina, part of the Pampas region. Capital: La Plata.