An organism of microscopic or submicroscopic size, specifically a bacterium or protozoan.
An organism that's also tiny to be noticed because of the unaided eye, specially a single-celled system, including a bacterium.
Any microscopic kind of life; a form of life also little to be seen because of the naked-eye; -- specifically put on bacteria, protozoa, yeasts, and similar organisms, esp. such are supposed to trigger infectious diseases.
A microscopic system, as a bacillus, bacterium, or vibrio; a microbe; a microzoary.