\begin{definition}[Definition:Power Series/Real Domain]
Let $\xi \in \R$ be a real number.
Let $\sequence {a_n}$ be a sequence in $\R$.
The series $\ds \sum_{n \mathop = 0}^\infty a_n \paren {x - \xi}^n$, where $x \in \R$ is a variable, is called a '''power series in $x$ about the point $\xi$'''.
\end{definition}