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

Cell division in eukaryotes is different from that in organisms without a nucleus (Prokaryote). There are two types of division processes. In mitosis, one cell divides to produce two genetically identical cells. In meiosis, which is required in sexual reproduction, one diploid cell (having two instances of each chromosome, one from each parent) undergoes recombination of each pair of parental chromosomes, and then two stages of cell division, resulting in four haploid cells (gametes). Each gamete has just one complement of chromosomes, each a unique mix of the corresponding pair of parental chromosomes.

The domain Eukaryota appears to be monophyletic, and so makes up one of the three of domains of life. The two other domains, Bacteria and Archaea, are prokaryotes and have none of the above features. Eukaryotes represent a tiny minority of all living things; even in a human body there are 10 times more microbes than human cells. However, due to their much larger size their collective worldwide biomass is estimated at about equal to that of prokaryotes. They are considered to have first developed approximately 1.6–2.1 billion years ago.