Protozoa are a diverse group of unicellular eukaryotic which are single celled complex organizms with a nucleus and organelles. Historically, protozoa were defined as single-celled organisms with animal-like behaviors, such as motility and predation.

Protozoa are mainly microscopic organisms, ranging in size from 10 to 52 micrometers, although some are significantly larger. Free-living forms are restricted to moist environments, such as soils, mosses and aquatic habitats, although many form resting cysts which enable them to survive drying. Many protozoan species are symbiontic, some are parasites, and some are predators of bacteria, algae and other protists. A protists is any eukaryoitic (an organism which possess a nucleus and membrane-bound organelles), which means it cells have a nucleus but it is not an animal, plant or fungus. It is primary a single-celled, microscopic organism which inhabit aquatic and/or moist enviornments. Known for extreme diversity, they can act like animals (protozoa), plants (algae), or fungi (slime molds), typically classified based on what they are not.

The protozoa continue to used to described single-celled protist, that are eukaryotes that are not animals, plants, or fungi which feed by heterotrophy. A heterotrophic organism is one which cannot produce its own food, but instead takes its nutrition from other sources, whereas an autotrophic organism is one that can produce their own food, such as plants, algae and many bacteria.