Archaebacteria

Definition
noun, singular: archaebacterium
Unicellular microorganisms in the domain Archaea, which is genetically distinct from bacteria and eukaryotes, and often inhabiting extreme environmental conditions.
Supplement
Examples of archaebacteria include halophiles (microorganisms that may inhabit extremely salty environments), methanogens (microorganisms that produce methane), and thermophiles (microorganisms that can thrive extremely hot environments).
They evolved separately from eubacteria and eukaryotes. They are similar with eubacteria in being prokaryotes and lacking distinct cell nucleus. They differ in terms of ribosomal structure, the possession of introns and in membrane structure or composition. They are similar to eukaryotes in ways that archaea possess genes and several metabolic pathways that are more closely related to those of eukaryotes: notably the enzymes involved in transcription and translation.
Word origin: New Latin, from Greek arkhaion, neuter singular of arkhaios, ancient + bacteria.
Related forms: archaeobacteria.
Synonym: archaea.

Compare: eubacteria.