What is defined as an organism that is so small it can only be seen with a microscope?

The term being defined as an organism that is so small it can only be seen with a microscope is "microorganism." This term encompasses a wide variety of tiny life forms, including bacteria, viruses, fungi, and protozoa, which are typically measured in micrometers (µm) or nanometers (nm). Microorganisms play crucial roles in various ecosystems, human health, and industry, which is why understanding them is essential in fields like pest management.

While "microbe" is often used interchangeably with "microorganism," it generally refers to the same group of organisms but is a broader term that can sometimes include more specific categories, such as bacteria. The terms "nanoorganism" and "submicrobial" do not align with the standard definition as they imply smaller or alternative classifications that are not typically recognized in the scientific community for the general context of organisms seen only with a microscope. Therefore, "microorganism" is the most accurate and encompassing term for this definition.