To be pedantic, if we're going to cite Unix history, then technically there is no such thing as a "hidden" file. As I understand, the original implementation of tools like ls would not show the current directory "." in the listing. The quickest way of doing this in C was checking whether dentry.name[0] == '.'

This also had the side effect of hiding entries that had names beginning with ".", and over time an implementation bug/quirk became accepted behaviour, and was post-hoc rationalised as "hidden" files.