Alphabet
Also known as: Σ
An alphabet $\Sigma$ is a finite nonempty set of symbols. Strings are finite sequences drawn from $\Sigma$; $\Sigma^*$ is every finite string including the empty one.
Coder view: the charset or the set of event names your machine understands. For a binary DFA, $\Sigma = {0,1}$. For a UI controller, $\Sigma$ might be {press, release, tick}.
Every formal language and every automaton is relative to some $\Sigma$. Change the alphabet and you change which strings even exist, before acceptance comes into play.