A microorganism, or microbe, is any organism too small to be seen by the naked eye. Bacteria, viruses, protozoa, and some fungi and algae are microorganisms.

A living entity which is visible only with the aid of a microscope.

a living organism too small to be seen with the naked eye. Bacteria, protozoans, viruses, microscopic algae and some types of fungi are all microorganisms.

any organism too small to be viewed by the unaided eye