Which term describes any organism of microscopic or submicroscopic size?

The term that describes any organism of microscopic or submicroscopic size is "microorganism." This classification encompasses a variety of life forms, including bacteria, viruses, fungi, and protozoa, which are not visible to the naked eye and require a microscope for observation. Microorganisms play essential roles in various ecosystems and can be both beneficial and harmful to humans.

While the terms "pathogen" and "bacteria" refer to specific types of microorganisms—the former indicating those that cause disease and the latter being one type of microorganism—the broader term "microorganism" includes all forms that fit the size criterion. Moreover, "macroorganism" refers to larger organisms that can be seen without a microscope, which does not apply to the microscopic definitions sought in this context. Therefore, "microorganism" is the most comprehensive and appropriate term for organisms of microscopic or submicroscopic size.