1. 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. 2. 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. 3. The division of or tendency to divide a society into subclasses, groups, or units. 4. Saturday Review
