\begin{definition}[Definition:Power Series/Center]
Let $\ds \sum_{n \mathop = 0}^\infty a_n \size {x - \xi}^n$ be a power series.
The point $\xi$ is the '''center''' of $\ds \sum_{n \mathop = 0}^\infty a_n \size {x - \xi}^n$.
\end{definition}