1 : an organism (especially a bacterium) that does not require air or free oxygen to live
2 : (microbiology) single-celled or noncellular spherical or spiral or rod-shaped organisms lacking chlorophyll that reproduce by fission; important as pathogens and for biochemical properties; taxonomy is difficult; often considered to be plants
3 : an organism (especially a bacterium) that requires air or free oxygen for life
