Demonym

He had to choose between "Earthling" and "Terran" for their demonym.

A demonym is the name or term for the residents of a particular locality.

A demonym is typically - but not always - derived from a placename, though it is equally true that many countries are named for their inhabitants.

The association of kiwi with New Zealand is so strong that 'Kiwi' is frequently used as a colloquial demonym for New Zealanders.

Why is it that people from the United States use American as their demonym?

The Logophile has my favourite demonym; I would write under it if he didn't.