Used mainly in science, especially in biology and microbiology discussions. In my biology class, we learned that an amoeba is a unicellular organism.
Pro Tip 2/3
Size Implication
Unicellular suggests something is very small, often microscopic. Even though it's unicellular, this organism can have a huge impact on its environment.
Pro Tip 3/3
Complexity
Emphasizes simplicity; such organisms typically do not have the complex structure of multicellular ones. Unicellular creatures like bacteria lack the complex organs found in humans.