Microorganism (noun)

1. A tiny living organism, such as a bacterium, fungus, or protozoan, that can only be seen with a microscope.
2. A single-celled organism that cannot be seen with the naked eye.

Origin:

From micro- + organism.

Examples:

1. The water sample contained a high number of microorganisms.
2. The scientist studied the microorganisms found in the soil.
3. Some microorganisms are beneficial, while others can cause disease.
4. The microorganism caused the fermentation of the bread dough.
5. The microorganism was essential in the production of yogurt.