Source : Webster's Revised Unabridged Dictionary (1913)

Archaean \Ar*ch[ae]"an\, a. [Gr. 'archai^os ancient, fr. 'archh`
   beginning.]
   Ancient; pertaining to the earliest period in geological
   history.

Archaean \Ar*ch[ae]"an\, n. (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.

   Note: This is equivalent to the formerly accepted term Azoic,
         and to the Eozoic of Dawson.

Source : WordNet®

archaean
     adj : used of the earliest known rocks [syn: {archean}]