The term 'unicellular critter' refers to a single-celled organism, such as the AMOEBA (6 letters) and ameba (5 letters). These organisms are among the simplest forms of life, consisting of a single cell that is capable of movement and feeding. They are found in a variety of habitats, from freshwater to saltwater, and can survive in extreme temperatures. Amoebas and amebas are classified as protists, which are eukaryotic organisms that lack the characteristics of plants, animals, and fungi.