Biology. A single-celled organism, esp. a protozoan; spec. a flagellate protozoan of the genus Monas, the former family Monadinidae (or Monadidae)…

a primordial or primitive organism; (in early use) †a microorganism (obsolete).

Esp. in the terminology of Haeckel: a cell-like mass of protoplasm lacking a nucleus, believed to be the most primitive form of living organism. Now…

With singular agreement: a kingdom comprising organisms with a relatively simple organization that are regarded as intermediate between or…

An organism of the kingdom Protista; esp. a single-celled eukaryotic organism; a protozoan or a unicellular alga.

Used by Bastian as a general name for the lowest forms of life, which cannot be assigned definitely to either the animal or vegetable kingdom.

= protist, n.