Hadean
The Hadean () is a geologic eon of the Earth pre-dating the Archean. It began with the formation of the Earth about 4.6 billion years ago and ended, as defined by the ICS, 4 billion years ago. , the ICS describes its status as ""informal"". Geologist Preston Cloud coined the term in 1972, originally to label the period before the earliest-known rocks on Earth. W. Brian Harland later coined an almost synonymous term, the "Priscoan period", from "priscus", the Latin word for "ancient". Other, older texts simply refer to the eon as the Pre-Archean.
Helladic chronology
Helladic chronology is a relative dating system used in archaeology and art history. It complements the Minoan chronology scheme devised by Sir Arthur Evans for the categorisation of Bronze Age artefacts from the Minoan civilization within a historical framework. Whereas Minoan chronology is specific to Crete, the cultural and geographical scope of Helladic chronology is mainland Greece during the same timespan (c.3200–c.1050). Similarly, a Cycladic chronology system is used for artifacts found in the Aegean islands. Archaeological evidence has shown that, broadly, civilisation developed concurrently across the whole region and so the three schemes complement each other chronologically. They are grouped together as "Aegean" in terms such as Aegean civilization.