A prokaryote is a single-celled organism that lacks a membrane-bound nucleus (karyon), mitochondria, or any other membrane-bound organelle. The word prokaryote comes from the Greek πρό (pro) "before" and καρυόν (karyon) "nut or kernel". Prokaryotes can be divided into two domains, Archaea and Bacteria, with the remainder of species, called eukaryotes, in the third domain Eukaryota.