Monera (Greek - μονήρης (monḗrēs), "single", "solitary") ( ) was a kingdom that contained unicellular organisms with a prokaryotic cell organization (having no nuclear membrane), such as bacteria. The taxon Monera was first proposed as a phylum by Ernst Haeckel in 1866. Subsequently, the phylum was elevated to the rank of kingdom in 1925 by Édouard Chatton. The last commonly accepted mega-classification with the taxon Monera was the five-kingdom classification system established by Robert Whittaker in 1969.