1. • noun - (psychology) a theory that reduces all mental phenomena to simple elements (sensations and feelings) that form complex ideas by association
   • noun - (chemistry) any theory in which all matter is composed of tiny discrete finite indivisible indestructible particles
2. • The ancient theory of Democritus, Epicurus, and Lucretius, according to which simple, minute, indivisible, and indestructible particles are the basic components of the entire universe.
   • A theory according to which social institutions, values, and processes arise solely from the acts and interests of individuals, who thus constitute the only true subject of analysis.
   • The division of or tendency to divide a society into subclasses, groups, or units.
   • The understanding of society as a set of independent wills, based on a belief in the priority of individual over cooperative values: “the kind of atomism and individualism that is implicit in what might be called the John Wayne conception of society and the individual” (Saturday Review).
3. • n - the theory that the universe is composed of simple, indivisible, minute particles

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