Let $(z_n)_{n\in\mathbb N}$ be a complex sequence. An accumulation point of $(z_n)_{n\in\mathbb N}$ is a complex number $\alpha$ such that, for every $\epsilon > 0$ the open disc $B(\alpha,\epsilon)$ contains infinitely many sequence members $z_n.$
More generally, an accumulation point of an infinite subset $U\subseteq\mathbb C$ is a complex number $\alpha$ such that, for every $\epsilon > 0,$ the open disc $B(\alpha,\epsilon)$ contains infinitely many elements of $U.$
