microorganism

Meanings

1. 1
   noun

   An organism that is too small to be seen by the unaided eye; especially, a single-celled organism, such as a bacterium.

   The cud is then reswallowed by the ruminant and further digested by specialized microorganisms that live in the rumen.

2. 2
   noun

   any organism of microscopic size

Etymology

From micro- + organism.