What is the term for any organism that is too small to be seen without a microscope?

The term "microorganism" refers specifically to organisms that are so small they cannot be seen with the naked eye and require a microscope for observation. This category includes bacteria, viruses, fungi, and some protozoa. Microorganisms play critical roles in various ecosystems, including both beneficial roles, such as in digestion and decomposition, and harmful roles, such as causing infections.

The other terms listed do not accurately describe organisms based solely on their size. For instance, "cell" is the basic unit of life but can include both microscopic and macroscopic entities. "Tissue" refers to a group of similar cells working together to perform a specific function, and this is only visible to the naked eye in larger organisms. "Pathogen" is a term used for microorganisms that cause disease; while all pathogens are microorganisms, not all microorganisms are pathogens. Thus, "microorganism" is the most appropriate choice to define organisms that are too small to be seen without a microscope.