Definition:Atom


 * Logic:
 * Atom: a concept at an irreducible level of simplicity, for example:
 * a simple statement in propositional logic


 * Measure theory:
 * Atom of a measure


 * Abstract algebra:
 * Atom of a $\sigma$-algebra
 * Irreducible Element of an integral domain


 * Physics:
 * Atom: the smallest piece of matter that can exist of a particular type of substance


 * Whatever this is:
 * Definition:Atomic Formula