What characteristic defines protozoa?

The characteristic that defines protozoa is that they are single-celled microscopic organisms. Protozoa belong to a diverse group of eukaryotic microorganisms, which means they have a complex cell structure with a defined nucleus. This single-celled nature allows protozoa to perform all necessary life functions within that one cell, including movement, reproduction, and response to environmental stimuli.

Protozoa can be found in various environments, primarily in water or as parasites, and they can exhibit a wide range of behaviors and forms. Their classification includes various types, such as amoebas, flagellates, and ciliates, all sharing the common trait of being microscopic and unicellular. This characteristic is essential for distinguishing protozoa from other biological organisms, such as fungi and plants, which are typically multicellular or have a different structure or function.