eukaryote, a cell or organism composed of cells that have a membrane-bound nucleus and organelles (mitochondria, chloroplasts; see cell, in biology) and genetic material organized in chromosomes in which the DNA is combined with histone proteins. Eukaryotes are contrasted with the prokaryotes (see Monera). Eukaryotes formed through the merger of prokaryotes, which predate them in the fossil record by some 2 billion years. In the five-kingdom system of classification, the eukaryotes have comprised the taxonomic kingdoms Protista, Fungi, Plantae (see plant), and Animalia (see animal). In a recently proposed system they are called the eukarya and classified as an overarching group (domain) above the kingdom level.

Any organism composed of one or more cells, each of which contains a clearly defined nucleus enclosed by a membrane, along with organelles (small, self-contained, cellular parts that perform specific functions). The organelles include mitochondria, chloroplasts, a Golgi apparatus, an endoplasmic reticulum, and lysosomes. All organisms except bacteria and archaea are eukaryotes; bacteria and archaea are prokaryotes.

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