The name Tertia is a girl's name of Latin origin meaning "third". Tertia and is often added to lists like Cool Girl Names from Ancient Cultures and discussed in our forums with posts like "This or That?------------girls!".
From the experts:
Tertia—which is occasionally used in the UK, but rarely in the US, would make an unconventional but interesting possibility for the third child in a family. Terza and Terzo are lively Italian female and male variations.
This is just one of several Roman numeral names that suddenly sound plausible--like Primo and Prima, Decimus, Quintus, etc.