latex symbol for "if and only if"

LaTeX defines \to as \rightarrow:

\let\to\rightarrow % fontmath.ltx

The other direction is \gets:

\let\gets\leftarrow

For \leftrightarrow you can define your own command, e.g. \biconditional:

\documentclass{article}
\let\biconditional\leftrightarrow
\begin{document}
\[ A \to B \biconditional C \gets D \]

\[ A \rightarrow B \leftrightarrow C \leftarrow D \]
\[ A \longrightarrow B \longleftrightarrow C \longleftarrow D \]

\[ A \Rightarrow B \Leftrightarrow C \Leftarrow D \]
\[ A \Longrightarrow B \Longleftrightarrow C \Longleftarrow D \]
\[ B \iff C \]
\end{document}

Remarks:

• \iff adds some extra space (from fontmath.ltx):

  \DeclareRobustCommand\iff{\;\Longleftrightarrow\;}
  

• The example also shows some other arrow variants.

You can use \Leftrightarrow or \Longleftrightarrow

\documentclass{article}
\begin{document}
$ \Leftrightarrow \Longleftrightarrow $
\end{document}

You might want to also bookmark this: https://oeis.org/wiki/List_of_LaTeX_mathematical_symbols

Double line:

• \iff (= \Leftrightarrow) or
• \longLeftrightarrow or
• \xLeftrightarrow (extensible, load the mathtools package)

Single line:

• \leftrightarrow or
• \longleftrightarrow or
• \xleftrightarrow (extensible, load the mathtools package)