The [[Domain]] of a [[Function]] $f : A\to B$ is the [[Set]] of all 'allowed' [[Pre-Image|Inputs]]. $\huge f : {\color{pink} {A}} \to B $ The [[Domain]] is typically part of the *definition* of $f$ rather than a derived property. The [[Domain]] of some function $f$ is sometimes denoted as: $\huge \begin{align} \op{dom} \, f &= A \\ \op{dom} (f) &= A \end{align} $