\begin{definition}[Definition:Indexing Set/Index]
Let $I$ and $S$ be sets.
Let $x: I \to S$ be a mapping.
Let $x_i$ denote the image of an element $i \in I$ of the domain $I$ of $x$.
Let $\family {x_i}_{i \mathop \in I}$ denote the set of the images of all the element $i \in I$ under $x$.
An element of the domain $I$ of $x$ is called an '''index'''.
\end{definition}