Single-celled microorganisms that often live in extreme environments. They look like bacteria but biochemically and genetically are very unlike bacteria.

<microbiology> This is a classification of odd bacteria that are neither prokaryotes nor eukaryotes, some scientists believe they represent a separate kingdom. The primary species is Archaebacteria, whose members fall in three categories: microbes that can live in extremely salty environments (halophiles), microbes that produce methane (metha [..]