The term that describes a single-celled organism is "unicellular." Unicellular organisms consist of just one cell, which carries out all the necessary functions for survival, including metabolism, reproduction, and response to the environment. They are incredibly diverse and can be found in various environments, including water, soil, and within other living organisms. Examples of unicellular organisms include bacteria, amoebae, and some species of algae. These organisms play crucial roles in ecosystems, participating in processes like nutrient cycling and serving as the foundation of food webs.