Is there a name for the set of single-character prefixes of words in a regular language

Question

This question concerns regular languages and regular expressions.

Suppose we have an alphabet $\Sigma$ and a language $\mathcal{L}$. For some algorithm, I am particularly concerned with the set of first symbols of all words in $\mathcal{L}$. So I was wondering whether there exists a name for such a set (defined below):

$$ A = \{ a \in \Sigma \mathrel{|} \exists w. aw \in \mathcal{L} \} $$

Any reference would be greatly appreciated.

Answer 1

Personally, I call it the set of first letters of the words of $L$. See Example 5.2, page 54 in these notes.