What is the definition of a microorganism?

A microorganism is typically defined as an organism that is microscopic, meaning it is too small to be seen with the naked eye. This classification primarily includes single-celled organisms, such as bacteria and fungi, which can be observed and studied through the use of microscopes. The defining characteristic of a microorganism is its size and structure, as well as its ability to carry out life processes, mostly at the cellular level, which encompasses a vast diversity of life forms that are crucial in various ecological and biological processes.

In contrast, multicellular organisms like plants and animals, while they also play significant roles in the ecosystem, do not fall under the category of microorganisms due to their size and complexity. Viruses are unique entities that lack the cellular structure of living organisms and are often debated in the context of biological classification; they are not considered living organisms because they cannot carry out metabolic processes independently. Macromolecules, such as proteins and nucleic acids, are essential components of living cells but do not qualify as organisms themselves. Thus, defining a microorganism is most accurately captured by its single-cell structure and its role in the microscopic world.