The name Hadean refers to a geologic period: the time prior to 3,800,000,000 years before the present -- 3,800MA. The geologist Preston Cloud coined the now somewhat outdated term in 1972, originally to label the period before the earliest known rocks. In the last decades of the 20th century geologists identified a few Hadean rocks from Western Greenland, Northwestern Canada and Western Australia.

The term Hadean seldom occurs. W. B. Harland has coined an almost synonymous term: the "Priscoan period".

The oldest known rock formations comprise somewhat altered sediments from Greenland dated around 3800MA by a volcanic dike that penetrated the rocks after they were deposited. Individual zircon crystals redeposited in sediments in Western Canada and the Jack Hills region of Western Australia are much older. The oldest dated zircons date from about 4400MA - very close to the hypothesized time of the Earth's formation . The Greenland sediments include banded iron beds. They contain possibly organic carbon and quite possibly indicate that photosynthetic life had already emerged at that time. The oldest known fossils (from Australia) date from a few hundred million years later.

The etymology of the name "Hadean" (from Hades) suggests underworld-like heat and confusion.