Definition:Biconditional

Definition
Material Equivalence is a binary connective written symbolically as $$p \iff q$$ whose behaviour is as follows:


 * $$p \iff q$$ is defined as $$\left({p \implies q}\right) \and \left({q \implies p}\right)$$

Thus, $$p \iff q$$ means:
 * "$$p$$ is true if and only if $$q$$ is true";
 * "$$p$$ is (logically) equivalent to $$q$$";
 * "$$p$$ is true iff $$q$$ is true".

"$$p \iff q$$" can be voiced "$$p$$ if and only if $$q$$".

Other names for this operator include:
 * Biconditional;
 * Logical Equivalence;
 * Logical Equality.

Boolean Interpretation
From the above, we see that the boolean interpretations for $$\mathbf A \iff \mathbf B$$ under the model $$\mathcal M$$ are:


 * $$\left({\mathbf A \iff \mathbf B}\right)_{\mathcal M} = \begin{cases}

T & : \mathbf A_{\mathcal M} = \mathbf B_{\mathcal M} \\ F & : \text {otherwise} \end{cases}$$

Complement
The complement of $$\iff$$ is the exclusive or operator.

Truth Table
The truth table of $$p \iff q$$ and its complement is as follows:

$$\begin{array}{|cc||c|c|} \hline p & q & p \iff q & p \oplus q \\ \hline F&F&T&F\\ F&T&F&T\\ T&F&F&T\\ T&T&T&F\\ \hline \end{array}$$

Semantics of Equivalence
The concept of material equivalence has been defined as:


 * $$p \iff q$$ means $$\left({p \implies q}\right) \and \left({q \implies p}\right)$$

So $$p \iff q$$ means: "If $$p$$ is true then $$q$$ is true, and if $$q$$ is true then $$p$$ is true." $$p \iff q$$ can be considered as a shorthand to replace the use of the longer and more unwieldy expression involving two conditionals and a conjunction.

If we refer to ways of expressing the conditional, we see that:


 * $$q \implies p$$ can be interpreted as "$$p$$ is true if $$q$$ is true," and
 * $$p \implies q$$ can be interpreted as "$$p$$ is true only if $$q$$ is true."

Thus we arrive at the usual way of reading "$$p \iff q$$" which is: "$$p$$ is true if and only if $$q$$ is true."

This can also be said as:


 * "The truth value of $$p$$ is equivalent to the truth value of $$q$$."
 * "$$p$$ is equivalent to $$q$$."
 * "$$p$$ and $$q$$ are equivalent."
 * "$$p$$ and $$q$$ are coimplicant."
 * "$$p$$ and $$q$$ are materially equivalent."
 * "$$p$$ is true exactly when $$q$$ is true."
 * "$$p$$ is true iff $$q$$ is true." This is another convenient and useful (if informal) shorthand which is catching on in the mathematical community.

Necessary and Sufficient
If $$p \iff q$$, we can say that "$$p$$ is necessary and sufficient for $$q$$".

This is a consequence of the definitions of necessary and sufficient conditions.

Notational Variants
Alternative symbols that mean the same thing as $$p \iff q$$ are also encountered:


 * $$p \leftrightarrow q$$;
 * $$p\ \texttt{EQ}\ q$$;
 * $$p = q$$;
 * $$p \equiv q$$.

It is usual to use "$$\iff$$", as then it can be ensured that it is understood to mean exactly the same thing when we use it in a more "mathematical" context. There are other uses in mathematics for the other symbols.