Archaea is a term that refers to a diverse group of single-celled organisms that inhabit extreme environments such as hot springs, deep-sea vents, or acid lakes. Synonyms for archaea include "archaebacteria," "ancient bacteria," "archaic microorganisms," and "archaebes." These microorganisms are distinct from bacteria and eukaryotes, possessing unique biochemical and genetic characteristics that allow them to thrive in extreme environments. Archaea play critical roles in biogeochemical cycles and ecological processes, such as nitrogen fixation, sulfur oxidation, and methane production. They are also essential for the production of certain commercial products, such as biofuels and antibiotics. Despite their importance, much remains to be learned about the diversity and ecological functions of these remarkable organisms.