archæan

from The Century Dictionary.

• Of or relating to the oldest period of geological time: a name proposed by J. D. Dana, and now generally adopted, for a series of crystalline schists and massive rocks lying underneath the most ancient fossiliferous stratified formations.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Ancient; pertaining to the earliest period in geological history.
• noun (Geol.) The earliest period in geological period, extending up to the Lower Silurian. It includes an Azoic age, previous to the appearance of life, and an Eozoic age, including the earliest forms of life.