A power series (centered at the origin) is an expression of the form:
$$\displaystyle \sum_{n=0}^{\infty} a_n x^n = a_0+a_1x+a_2x^2+a_3x^3+\ldots$$ where $x$ is a variable and each $a_i$ is a fixed number. For example,
$\displaystyle \sum_{n=0}^\infty \frac{x^n}{n!},\text{ or }\sum_{n=0}^\infty \frac{x^n}{n+1}.$
Definitions:
The set of $x$ where the series converges is called the interval of convergence, and is an interval from $-R$ to $R$, where $R\ge0$ is a called the radius of convergence.
The interval of convergence may include one, both, or no endpoints, for which we have to check separately.
Special cases:
If $R=0$, then the series converges at the single point $x=0$;
If $R=\infty$, then the series converges on the entire real line.