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Direct from Latin with the same meaning: third. As a bonus, we English speakers are able to use it as a noun and an adjective. Not only that this word is useful in a variety of disparate fields: chemistry, geology, fines arts (tertiary colors), medicine, education and...you won’t believe this...a lay association of certain Christian monasteries! A Franciscan tertiary.

And, even though the Earth is third, not related, despite visual similarities, to terrestrial or terracotta.