An axiom is a sentence admitted as a starting assumption of a formal theory. An axiom schema specifies a collection of such sentences, usually one for each formula of an allowed form. A model of a first-order theory satisfies all its axioms; a relative consistency construction can show that the other axioms do not imply a particular omitted one.
An axiom set is the collection of formulas available as axioms in a specified formal proof system. It may be finite or infinite, or specified by an axiom scheme.
For an independent axiom set, semantic entailment of any one axiom from the remaining axioms fails. By the completeness theorem, this is equivalent to no member being formally derivable from the others in a complete calculus. To establish independence it suffices to give, for each axiom, a first-order structure satisfying all the other axioms and failing that one.
The usual associative, identity, two-sided inverse and commutativity axioms form an independent axiom set. A three-element commutative unital operation with self-inverses can fail associativity; a constant-zero operation can satisfy the other three axioms while failing the identity condition; natural-number addition fails the inverse condition; and a noncommutative group fails commutativity. Each example must be checked against all three retained axioms.
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The term "axiom" generally refers to a fundamental principle or starting point that is accepted as true without proof, serving as a foundation for further reasoning or arguments. Axioms are commonly used in mathematics and logic to establish a framework for a theory or system. In mathematics, for example, axioms are the basic assumptions upon which theorems are derived. For instance, in Euclidean geometry, the parallel postulate is an axiom that leads to various geometric propositions.
