Domain of definition
From HandWiki
of a function
The set on which the function considered is given, that is, the collection $X$ of all elements $x$ with each of which the given function $f$ associates an element $y$ of some set $Y$. Thus, if $f:X \to Y$, then $X$ is called the domain of definition of $f$.
Comments
The set of values of $f$, i.e. $f(X)$ if $X$ denotes the domain of definition, is called the range of values of $f$.
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