The term 'single-celled critter' can be summarized by the crossword solution 'ameba' (5 letters). An ameba is a single-celled organism that belongs to the protozoa family. It is a simple organism that moves by extending and retracting its pseudopods, or false feet. Amebas are found in many different environments, including freshwater, saltwater, and soil. They feed on bacteria and other small organisms.