An air-breather was anyone who inhaled gaseous air rather than water. It was a term used to describe those that remained to live on the "surface places" of planet Argo. According to Ruling Tribunal of the Aquans, the ancient Aquan records said that air-breathers were the enemies from the surface, who never came in peace, and whom the Aquans expected spies from for a long time.

In 2269, Cadmar referred to James T. Kirk and Spock as air-breathers, telling them to leave. When told they were surface dwellers from another planet, Cadmar dismissed their lies, fearing they were there to destroy the Aquans. (TAS: "The Ambergris Element")

According to the first draft of the script, dated 5 August 1973, the term "Agranoid" was used in place of "air-breather". In unused dialog appearing in the first draft it was explained by Rila that "The race has not been seen since the time of the Ancients, when the surface places disappeared." Domar, however, believed that "The survivors have been in hiding – awaiting the time when they are again strong enough to destroy us."

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