monad

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Philosophy An indivisible, impenetrable unit of substance viewed as the basic constituent element of physical reality in the metaphysics of Leibniz.
2. n. Biology A single-celled microorganism, especially a flagellate protozoan of the genus Monas.
3. n. Chemistry An atom or a radical with valence 1.

Century Dictionary and Cyclopedia

1. n. In metaphysics, an individual and indivisible substance. The word was introduced into philosophy by Giordano Bruno to denote the minimum parts of substances supposed by him to be at once psychical and material. In the philosophy of Leibnitz the conception of the monad is that of an absolutely unextended substance existing in space, its existence consisting in its activities, which are ideas; and the universe was conceived by him as made up of such existences. The history of each monad follows an internal law, and all interaction between the monads is excluded; but there is a preestablished harmony between these laws for the different monads. (See Leibnitzian.) The Leibnitzian theory of the monad was, in many particulars, revived by Hermann Lotze.
2. n. In biology:
3. n. Any simple single-celled organism. The name covers a great many similar but not necessarily related unicellular organisms, some of which are monads in sense , others being plants; others again are free flagellate cells representing an embryonic condition of some other organism or of wholly indeterminate character.
4. n. 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.
5. n. In chem., an element whose atoms have the lowest valence or atomicity, which valence is therefore taken as unity.
6. In chem. and biology, of or pertaining to monads; of the nature of a monad; monadi-form.

Wiktionary

1. n. An ultimate atom, or simple, unextended point; something ultimate and indivisible.
2. n. mathematics, computing A monoid in the category of endofunctors.

GNU Webster's 1913

1. n. An ultimate atom, or simple, unextended point; something ultimate and indivisible.
2. n. (Philos. of Leibnitz) 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.
3. n. (Zoöl.) One of the smallest flagellate Infusoria; esp., the species of the genus Monas, and allied genera.
4. n. (Biol.) A simple, minute organism; a primary cell, germ, or plastid.
5. n. (Chem.) An atom or radical whose valence is one, or which can combine with, be replaced by, or exchanged for, one atom of hydrogen.

WordNet 3.0

1. n. (chemistry) an atom having a valence of one
2. n. (biology) a single-celled microorganism (especially a flagellate protozoan)
3. n. a singular metaphysical entity from which material properties are said to derive