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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.
This entry comes from Encyclopædia Britannica Concise. For the full entry on eukaryote, visit Britannica.com.