Noun
1. The branch of biology that deals with microorganisms, especially their effects on man and other living organisms. (wiktionary.com)
2. Refers to the study of microscopic organisms, which are defined as any living organism that is either a single cell (unicellular), a cell cluster, or has no cells at all (acellular). Microbiology is a broad term which includes virology, mycology, parasitology, bacteriology, immunology and other branches. (wikipedia.com)
3. The study of organisms that are too small to be seen with the naked eye, such as bacteria, viruses and yeasts. (biology-online.org)
Word origin: From Greek μῑκρος, mīkros, “small”; βίος, bios, “life”; and -λογία, -logia