It seems that Microsoft has chosen the Santorini name as the code name for Windows Lite, a new version of OS that is supposed to be lightweight and is intended to compete with Google's Chrome OS.
According to one report from WindowsCentral, Microsoft employees say that Windows Lite will be called Santorini (yes, from the Greek island of Santorini). It is not yet known if the name Santorini will be what the company will use for the final product.
The name Santorini was given by Microsoft employees because Lite does not represent the product as it is not a smaller version of Windows. It is a completely new OS designed for lightweight computer systems. Most likely the final product is not even called Windows.
In fact, Windows Lite is aimed specifically at new devices that do not require fully equipped Windows, as in the case of Andromeda, Microsoft's ambitious dual-screen design that has not yet seen the light of day.
It is not clear if Santorini is in any way representative of Windows Lite, but since it is an island name, there is a possibility that Microsoft did not take it just because it is auspicious.
Recent rumors have suggested that Microsoft could also use Windows Lite to test some features that would later be available in all versions of Windows 10, such as a simplified Start menu without tiles, etc. According to reports, Microsoft is planning abandon tiles due to their low acceptance, so the debut of Windows Lite could ultimately be a preview of other important changes for Windows 10 as a whole.