Biology .

any simple, single-celled organism.

any of various small, flagellate, colorless ameboids with one to three flagella, esp. of the genus Monas.

Chemistry . an element, atom, or group having a valence of one. Compare dyad ( def. 3 ) , triad ( def. 2a ) .

Philosophy .

(in the metaphysics of Leibniz) an unextended, indivisible, and indestructible entity that is the basic or ultimate constituent of the universe and a microcosm of it.

(in the philosophy of Giordano Bruno) a basic and irreducible metaphysical unit that is spatially and psychically individuated.

any basic metaphysical entity, esp. having an autonomous life.

a single unit or entity.

noun

atom , one , unit