The prokaryotes (pronounced /proʊˈkæri.oʊts/ or /proʊˈkæriəts/) are a group of organisms that lack a cell nucleus (= karyon), or any other membrane-bound organelles. They differ from the eukaryotes, which have a cell nucleus. Most are unicellular, but a few prokaryotes such as myxobacteria have multicellular stages in their life cycles. The word prokaryote comes from the Greek πρό- (pro-) "before" + καρυόν (karyon) "nut or kernel".
The prokaryotes are divided into two domains: the bacteria and the archaea. Archaea were recognized as a domain of life in 1990. These organisms were originally thought to live only in inhospitable conditions such as extremes of temperature, pH, and radiation but have since been found in all types of habitats.