The three domains refer to the three primary classifications of cellular life forms based on evolutionary relationships between organisms. The three domains are:

• Archaea: This domain includes single-celled organisms that are prokaryotic, meaning they lack a cell nucleus. Archaea are known for their ability to survive in extreme environments, such as hot springs and deep-sea hydrothermal vents/Unit_1%3A_Introduction_to_Microbiology_and_Prokaryotic_Cell_Anatomy/1%3A_Fundamentals_of_Microbiology/1.3%3A_Classification_-_The_Three_Domain_System).

• Bacteria: This domain also includes single-celled organisms that are prokaryotic. Bacteria are found in a wide range of environments and play important roles in many ecological processes/Unit_1%3A_Introduction_to_Microbiology_and_Prokaryotic_Cell_Anatomy/1%3A_Fundamentals_of_Microbiology/1.3%3A_Classification_-_The_Three_Domain_System).

• Eukarya: This domain includes organisms that have a cell nucleus and other membrane-bound organelles. Eukarya includes a wide range of organisms, from single-celled protists to multicellular animals and plants/Unit_1%3A_Introduction_to_Microbiology_and_Prokaryotic_Cell_Anatomy/1%3A_Fundamentals_of_Microbiology/1.3%3A_Classification_-_The_Three_Domain_System).

The three-domain system was introduced by Carl Woese, Otto Kandler, and Mark Wheelis in 1990. This classification system recognizes the fundamental divide between the two prokaryotic groups, insofar as Archaea appear to be more closely related to eukaryotes than they are to other prokaryotes – bacteria-like organisms with no cell nucleus.