What term is used for organisms that are too small to be seen without a microscope?

Microorganisms are defined as organisms that are so small that they cannot be seen with the naked eye and require a microscope for observation. This category includes various life forms such as bacteria, viruses, fungi, and some algae and protozoa. The term encompasses a wide range of sizes and shapes, but they all share the characteristic of being microscopic.

While bacteria are indeed a type of microorganism, the term "microorganisms" is broader, including all types in this size category. Macroorganisms, on the other hand, are large enough to be seen without assistance, and multicellular organisms specifically refer to those made up of multiple cells, which typically can be observed without a microscope. Thus, "microorganisms" is the most accurate term for organisms that are generally too small to be seen without a microscope.