Final answer:
In World War I, an 'ace' was defined as a pilot who shot down five enemy planes, with each victory confirmed by at least three witnesses.
Explanation:
An 'ace' in World War I was a pilot who brought down five enemy planes, with each victory confirmed by at least three witnesses. For example, Lieut. Rene Fonck ended the war with seventy-five official aerial victories, making him one of the top aces of the war.
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