A eukaryote is an organism with a complex cell or cells, in which the genetic material is organized into a membrane-bound nucleus or nuclei.
Eukaryotes (also spelled "eucaryotes") comprise animals, plants, and fungi—which are mostly multicellular - as well as various other groups that are collectively classified as protists (many of which are unicellular).
In contrast, prokaryotes are organisms, such as bacteria and archaea, that lack nuclei and other complex cell structures.
Eukaryotes share a common origin, and are often treated formally as a superkingdom, empire, or domain.
In the domain system, eukaryotes have more in common with archaean prokaryotes than bacterial prokaryotes.