biconditional the logical operator, usually written with a triple-bar sign (S S) or a doubleheaded arrow (Q), used to indicate that two propositions have the same truth-value: that either both are true or else both are false. The term also designates a proposition having this sign, or a natural language expression of it, as its main connective; e.g., P if and only if Q. The truth table for the biconditional is The biconditional is so called because its application is logically equivalent to the conjunction ‘(P-conditional-Q)-and-(Q-conditional-P)’. See also TRUTH TABLE. R.W.B.