What is the term for a living organism that cannot be seen by the naked eye?

The term "micro-organism" refers specifically to living organisms that are so small that they are not visible without the aid of a microscope. This category includes various forms of life such as bacteria, archaea, viruses, fungi, and protozoa. Given their size, they play crucial roles in ecosystems, human health, and many biotechnological applications.

In contrast, a "macro-organism" would refer to larger organisms, typically those that can be seen with the naked eye, such as plants, animals, and humans. "Microbe" is a broader term that can encompass any microscopic organism, including bacteria and viruses, but it is often used informally and may not specifically denote only those organisms that are microscopic in size. Lastly, "sub-microscopic organism" is not a commonly used term in biology and may imply organisms that are even smaller than microbes, which typically does not apply to most living organisms recognized in microbiology.

Therefore, "micro-organism" is the most accurate and widely accepted term for organisms that cannot be seen without magnification.