This is the standard limit definition of the complex exponential: $\displaystyle\lim_{n\to\infty}\left(1+\frac{z}{n}\right)^n = e^z$ for all $z\in\mathbb{C}$. This generalises the real limit $\lim_{n\to\infty}(1+x/n)^n = e^x$ to the complex plane.