An organism that can only be seen with a microscope is called a microscopic organism or a microorganism.

Here are some examples of microorganisms:

* Bacteria: Single-celled organisms that can be found in many environments.

* Viruses: Not technically living organisms, but they are microscopic and can cause disease.

* Protozoa: Single-celled organisms that are more complex than bacteria.

* Fungi: A diverse group of organisms that can be microscopic (like yeasts) or macroscopic (like mushrooms).

* Algae: Plant-like organisms that can be single-celled or multicellular.

Let me know if you have any other questions!