From Greek 'athanasia' meaning immortality, used among early Christians and Greek Orthodox saints.
Athanasia is one of the great philosophical names of the Greek tradition, constructed directly from *athanasia* — immortality — itself built from *a-* ('not') and *thanatos* ('death'). It is the feminine form of Athanasius, most famously borne by Saint Athanasius of Alexandria, the fourth-century theologian whose fierce defense of Trinitarian doctrine against Arianism earned him the epithet 'Athanasius contra mundum' — Athanasius against the world. The name thus carries both the lightness of linguistic poetry and the gravity of one of Christianity's most consequential doctrinal battles.
The feminine form Athanasia was relatively common in the Byzantine world, where the Greek Orthodox Church maintained an active cult of saints bearing the name. Saint Athanasia of Aegina, a ninth-century abbess who survived Viking raids, rebuilt her community, and eventually died in Constantinople, is among the most venerated bearers. The name appears in Byzantine imperial genealogies and in the records of Orthodox monasteries throughout Greece, the Balkans, and the eastern Mediterranean.
In Greece itself, Athanasia has never entirely left use — it remains a living name, often shortened to Thanas or Sia in everyday speech. For English-speaking parents, Athanasia is a significant undertaking — six syllables, three of them open vowels — but it rewards the commitment. It is one of very few given names that is literally a definition: to name a child Athanasia is to name her 'immortality.' Few names carry that kind of direct semantic charge, and the softened nicknames Sia or Nasia bring it gently to earth.