Protozoa (from the Greek words proton, meaning "first", and zoa, meaning "animals") is a subkingdom of microorganisms that are classified generally as unicellular non-fungal eukaryotes. Protozoans are a major component of the ecosystem. The word protozoan is originally an adjective and is used as a noun. While there is not any exact definition for the term protozoan, most scientists use the word to refer to a unicellular heterotrophic protist, such as the amoeba and ciliate.