The conjunction of two conditionals, the antecedent of each of which is the consequent of the other; that is, any statement of the form: "P if and only if Q." Although they may have other us [..]

An "If-Then" Statement that is true in both directions.  Sample conditional statement: pq. This be thought of as If p, then q AND If q then p. It is often presented as p "if and only if" q (sometimes shown as iff). The biconditional statement is only true when both p and q have the same truth values.