In lecture we wrote the following:
Generalised Binomial Theorem: For any $\alpha \in \mathbb{C}$ holds
$$(1+x)^\alpha = \sum_{k=0}^\infty \binom{\alpha}{k} x^k$$
where $x$ is a formal variable or $x \in \mathbb{C}$ with $\lvert x \rvert \le 1$.
But what is a formal variable? Does that mean that I can, for example, plug in a formal power series $A(z)$ for $x$?