A unicellular organism is a single-celled organism, such as an AMOEBA (6 letters), monad (6 letters) or ameba (5 letters). These organisms are made up of only one cell and are capable of performing all the necessary functions of life, such as respiration, metabolism, and reproduction. Unicellular organisms are found in all three domains of life: Archaea, Bacteria, and Eukarya. They are the most abundant type of organism on Earth and can be found in almost any environment.