bacterium, bacillus Definition

bacteria
plural noun

bacterium
1. biol.
An extremely diverse group of microscopic and usually single-celled organisms that occur in soil, water and air, including many parasitic species that cause numerous infectious diseases. See also prokaryote.

Derivative: bacterial
adj

Etymology: 19c: from Greek bakterion little stick.

Info:

Although both are commonly used to refer to microscopic organisms that cause disease, bacillus is, strictly speaking, correctly used only when referring to members of a particular group of bacteria, many of which are harmless.