The Collaborative International Dictionary of English v.0.48:

Archaean \Ar*ch[ae]"an\, a. [Gr. 'archai^os ancient, fr. 'archh`
   beginning.]
   Ancient; pertaining to the earliest period in geological
   history.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   Note: This is equivalent to the formerly accepted term Azoic,
         and to the Eozoic of Dawson.
         [1913 Webster] Archaeobacteria

WordNet (r) 3.0 (2006):

archaean
    adj 1: of or relating to the earliest known rocks formed during
           the Precambrian Eon [syn: archean, archaean]