monad

An indivisible, impenetrable unit of substance viewed as the basic constituent element of physical reality in the metaphysics of Leibniz.

A single-celled microorganism, especially a flagellate protozoan formerly classified in the taxonomic group Monadina.

In metaphysics, an individual and indivisible substance.

In biology:

Any simple single-celled organism.

In zoology, specifically, a flagellate infusorian; one of the Infusoria flagellata, characterized by the possession of one or two long whip-like flagella, and generally exhibiting an endoplast and a contractile vacuole. The word in this sense is derived from the name of the genus Monas.

In chem., an element whose atoms have the lowest valence or atomicity, which valence is therefore taken as unity.

In chem. and biology, of or pertaining to monads; of the nature of a monad; monadi-form.

An ultimate atom, or simple, unextended point; something ultimate and indivisible.

The elementary and indestructible units which were conceived of as endowed with the power to produce all the changes they undergo, and thus determine all physical and spiritual phenomena.