I've seen the term "configuration language" come up during the course of studying for the CSDA certification exam.
The formal definition provided is:
Configuration languages create files usually read and interpreted only once, during initialization
I also know that one use of a configuration language is to create files that set the initial state of an application - this is a question on the practice exams.
What are some examples of configuration languages?
Configuration languages are typically declarative. The most common example are Windows INI files, which are processed with using an API provided by Microsoft (or one of the many alternative implementations).
On Unix, many files in /etc follow some configuration language. A simple one would be /etc/inittab (configuration language for /sbin/init), a more complex one is sendmail.cf (configuration language for sendmail). Another example is PAM configuration (on Linux, typically in /etc/pam.d).