"Custom module" is, in Drupal jargon, a module that is developed for a specific website, and that is not generic enough to be generally used in other sites. An example of custom module is the Drupal.org customizations module, which contains code very specific for Drupal.org.
custom-module doesn't have any meaning because:
- A question tagged custom-module would not get a different answer, as the code used for a custom module is not different from the same code used from a module that is hosted on Drupal.org, and which is not a custom module.
- There isn't an expert of custom modules.
I have removed the tag from questions, but it gets periodically added to new questions.