What is defined as any organism of microscopic or submicroscopic size?

The term that describes any organism of microscopic or submicroscopic size is "microorganism." This category encompasses a wide variety of life forms, including bacteria, viruses, fungi, and protozoa, among others. Microorganisms can only be seen under a microscope due to their small size, which typically ranges from a few nanometers to a few micrometers.

Other terms, while related, have more specific definitions. A bacterial culture refers specifically to a population of bacteria that has been grown in a lab setting, thus not covering the entire range of microscopic organisms. A virus, meanwhile, is a type of microorganism that is submicroscopic and can only reproduce within a host cell, but it does not encompass all microscopic life. Pathogen is a broader term that refers to any microorganism that can cause disease, which includes bacteria and viruses, but again does not capture the full spectrum of microscopic organisms. Hence, "microorganism" is the most comprehensive and accurate answer to the question.