The term 'single-celled organism' refers to an organism composed of a single cell. These organisms are also known as unicellular organisms and are the most basic form of life on Earth. They are found in all three domains of life, including bacteria, archaea, and eukaryotes. Single-celled organisms are capable of performing all the necessary functions of life, such as metabolism, reproduction, and growth. They are typically much smaller than multicellular organisms, and can range from 0.1 to 5 micrometers in size.