A eukaryote ( ) is any organism whose cells contain a nucleus and other structures (organelles) enclosed within membranes. Eukaryotes are formally the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, enclosed by the nuclear envelope, which contains the genetic material. The presence of a nucleus gives eukaryotes their name, which comes from the Greek (eu, "well") and (karyon, "nut" or "kernel"). Most eukaryotic cells also contain other membrane-bound organelles such as mitochondria or the Golgi apparatus. In addition, plants and algae contain chloroplasts. Many unicellular organisms are eukaryotes, such as protozoa. All multicellular organisms are eukaryotes, including animals, plants and fungi.