Archaebacteria are unicellular microorganisms in the Archaea domain. Archaea is one of three domain groupings, along with Bacteria and Eukarya. Domain means the highest taxonomic ranking of organisms.
The word "archaebacteria" derives from the Greek word "arkhaion." It is the neuter singular of the word "arkhaios," which is a combination of "ancient" and "bacteria."
Many archaebacteria thrive under extreme conditions, such as in sulfur pools or in rock deep inside the Earth. They even exist on the ocean floor, though they lack oxygen and sunlight there.
Examples of archaebacteria include thermophiles, microorganisms that thrive in very hot environments, and halophiles, microorganisms that thrive in highly salty environments.