Any microscopic or very small organism considered (or at one time considered) to belong to the plant kingdom; esp. a microscopic alga. Cf. microzoary…

A microscopic animal organism; (later) spec. a microscopic metazoan. Usually in plural.

= microzoon, n.

With plural agreement. Originally: fungi and bacteria (considered as a division of the vegetable kingdom). Now: spec. fungi, esp. microscopic…

Zoology and Biology. A very minute germ or organism.

An extremely small living organism, a microorganism; esp. a bacterium causing disease or fermentation. Also in extended use.

An organism so small as to be visible only under a microscope; esp. a bacterium, fungus, or alga.

A relatively or abnormally small zooid (in its earlier and wider sense).

= microbe, n. Usually in plural.

colloquial (originally U.S.). A harmful microorganism or virus; a germ. Cf. superbug, n. 1.

Biology. A large spore (usually the larger of two types of spore) produced by various protoctists.

A microscopic animal organism; = microzoon, n.

A microscopic animal organism; = microzoon, n.

a vegetable organism which causes fermentation.

Originally: the causative agent or source of a disease, esp. an infectious disease. In later use: a bacterium or other microorganism, esp. when…

New Zealand colloquial. In singular, chiefly with the. Originally: a germ. In later use: a minor illness; a cold, etc.

A microorganism, or (in early use) a substance, used to restore or modify the composition of the microflora of a compartment of the body (esp. the…