Final answer:
Microorganisms are living organisms that are generally too small to be seen without a microscope.
Step-by-step explanation:
Microorganisms (or microbes) are living organisms that are generally too small to be seen without a microscope. They include a wide range of organisms, such as bacteria, fungi, viruses, and protozoa. Microorganisms differ from each other not only in size, but also in structure, habitat, metabolism, and many other characteristics.