What term is used to describe an organism made up of a single cell?

The term that describes an organism made up of a single cell is "unicellular." Unicellular organisms consist of one cell that performs all the necessary functions of life. This includes processes such as metabolism, growth, and reproduction, all occurring within that single cell. Examples of unicellular organisms include bacteria, yeast, and many protozoa.

In contrast, multicellular organisms are composed of multiple cells that may specialize in different functions, which allows for more complex structures and systems. The term "polycellular" is not commonly used in biological contexts and does not specifically denote single-celled organisms. The term "microscopic" refers to the size of an organism and can include both unicellular and multicellular organisms that are too small to be seen with the naked eye. Therefore, "unicellular" is the precise term for organisms made up of just one cell.