The island is referred to, in general, as Thira, but it is also known by other names in the sources. The very ancient name Stronghyle (the Round One), has its parallel in the Italian volcanic island of Stromboli, which was also called Stronghyle by ancient Greeks. Another name, again mentioned authors, was Kalliste (the Most Beautiful), no doubt referring to the matural beauties of the island. The name Thira itself derives from Thiras, son of Autesion, who colonizes the island together with the Venetians who ruled the Cyclades during the thirteenth, fourteenth and fifteenth centuries AD- Today, Thira, is more known as Santorini, which was apparently taken from a chapel dedicated to St. Irene in one of the bays where the Venetians moored their ships. There is a believes that the name came from the chapel in Perissa or from others, that came from the chapel in Rivas of Thirassia.
Calliste or Kallisti - According to tradition, the Island was first called Kalliste, meaning "the most beautiful". (Kalliste is the female for the best, while kallistos is the male. Greeks usually give female names to islands).
By that time the island was inhabited by Phoenicians. Kadmos the son of Agenor, stopped at Kalliste during one of his wanderings in search of Europe. He left behind a group of Phoenicians who were accompanying him and among them was his kinsman Membliaros. These Phoenicians occupied the island until the arrival of Theras.
Thira (or Thera) - In antiquity the island's name was Thera. According to tradition, the island was named after Theras, son of Autesion, who was the descendant of Polyneikes. Theras, having brought with him a band of Mycaenians from Sparta, made the island a colony of the Lacedaemonians. Before Theras, the island was occupied by Phoenicians and it was called "Kalliste".
(Although the island was named after Theras, the name Thera -- without the final "s" -- is a female name which is usual for most of the Hellenic islands.)
Strogili - Another ancient name referring to Santorini was Strogili.
Strogili means the "rounded". This name was given because of the round shape of the group of islands which form Santorini.
Santorini - The name Santorini is much more recent and is the name that Hellenes (Greeks) use nowadays for this group of islands. Nevertheless the name "Thera" is also used.
The name Santorini, which has a Latin root, was given by the Venetians who, after the Fourth Crusade (1204), had dominion over the islands. The origin of the name was the Chapel of Aghia Irini (Santa Irini - Santorini) which was built on Therasia. The Chapel was situated on a small bay where the Venetians moored their boats.