Which term describes an organism made of one cell?

The term that describes an organism made of one cell is "unicellular." This classification is used to identify organisms, such as bacteria, protozoa, and some algae, that exist as a single cell. Unicellular organisms carry out all necessary life processes within that one cell, including metabolism, reproduction, and response to their environment.

In contrast, multicellular organisms are composed of multiple cells that may be specialized for different functions, working together to sustain the life of the organism as a whole. Terms like polycellular and pluricellular are not commonly used in scientific literature; they are often used informally or incorrectly when referring to multicellular organisms. Therefore, the specificity of "unicellular" as a term captures the essence of these single-celled organisms effectively.