1) A genus of gram-positive, aerobic bacteria. Most species are free-living in soil and water, but the major habitat for some is the diseased tissue of warm-blooded hosts.

2) A genus of nonmotile acid-fast aerobic bacteria of the family Mycobacteriaceae that are usually slender and difficult to stain and that include the causative agents of tuberculosis (M. tuberculosis) and leprosy (M. leprae) as well as numerous purely saprophytic forms.

3) Any bacterium of the genus Mycobacterium or a closely related genus.