A unicellular protist is part of Eukaryota domain. It is very hard to classification protists because they do not necessarily have much in common, also, they are not an animal, plant or fungus. However, it is a eukaryotic organism that has a cell with nuclei and relatively simple level of organization. In some systems of biological classification, like five-kingdom scheme the protists form a kingdom called Protista (unicellular or unicellular-colonial organisms with no tissues).