A World's Fair was an exhibition on Earth.
In 1873, the Vienna Exposition was a World's Fair. It was visited by the Eighth Doctor and Mary Shelley. The Doctor described it to Mary as containing all "the trade and advancement of the Western world". (AUDIO: The Silver Turk)
In 1889, David Clayton visited a World's Fair. (PROSE: The Death of Art)
Valarie Lockwood and Roanna went on a date to the 1893 World's Fair, held in Chicago. While there, they were approached by Hayden Lockwood, claiming to be Valarie's husband. This commenced a temporal adventure involving the Lockwood Foundation, a mutatory virus, and a villain previously faced by Valarie and the Eleventh Doctor. (AUDIO: All's Fair)
In 1937, a World's Fair was held in Paris. Guernica was commissioned by the Spanish Republican government for their pavilion. (PROSE: History 101)