A binary logical connective, denoted
\[\begin{equation*}
P \iff Q \qor P \equiv Q,
\end{equation*}
\]
and read
| “\(P\) if and only if \(Q\)” | |
| or | “\(P\) is equivalent to \(Q\)”, |
that is ‘true’ if and only if
\(P\) and \(Q\) are both either ‘true’ or ‘false’,
which is equivalent to
\[\begin{equation*}
(P \implies Q) \> \land \> (Q \implies P).
\end{equation*}
\]