Archaea

n. group of microorganisms called bacteria that are different in molecular structure, archaebacterium

The Archaea , or archaebacteria, are a major group of microorganisms. Like bacteria, archaea are single-celled organisms that lack nuclei and are therefore prokaryotes, classified in kingdom Monera in the traditional five-kingdom taxonomy. Although there is still uncertainty in the phylogeny, Archaea, Eukaryota and Bacteria are the fundamental classifications of what is called the three-domain system. Although their prokaryotic features are diagnostic of that clade, archaea are more closely related to eukaryotes than to bacteria. To account for this, archaeans and eukaryotes are grouped together in the clade Neomura, which is thought to have arisen from gram-positive bacteria. Archaea were originally described in extreme environments, but have since been found in all habitats and may contribute up to 20% of total biomass.
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A unique group of microorganisms. They are called bacteria (Archaeobacteria) but they are genetically and metabolically different from all other known bacteria. They appear to be living fossils, the survivors of an ancient group of organisms that bridged the gap in evolution between bacteria and the eukaryotes (multicellular organisms). The name Archaea comes from the Greek archaios meaning ancient.

A prokaryote kingdom that has not diverged much from the ancestral prokaryote stock. Contemporary species of Archeabacteria live in extreme conditions. The three major groups are halobacteria, sulphobacteria and methanogens. All other prokaryotes are grouped in Eubacteria.
Archaea

prokaryotic organisms evolutionarily distinct from bacteria; their gene expression pattern (transcription and translation) is more like the eukaryotic pattern.