monad

Concise Oxford English Dictionary © 2008 Oxford University Press:
monad /ˈmɒnad, ˈməʊ-/

▶noun

• 1 technical a single unit; the number one.

• 2 Philosophy (in the philosophy of Leibniz) an indivisible and hence ultimately simple entity, e.g. an atom or a person.

• 3 Biology, dated a single-celled organism or a single cell.

– derivatives
monadic adjective,
monadism noun (Philosophy).

– origin C17: via late L. from Gk monas, monad- ‘unit’, from monos ‘alone’.