Is there any convention about the name to use for a file containing a class definition?
The only part of the Drupal coding standards that talks of filenames doesn't say how to name PHP files containing class definitions.

Files containing PHP code, which are not the main module file, or the installation file, usually use the extension .inc, in Drupal 7 and previous versions. As there isn't any possibility of conflict between files used from different modules, you can decide the rest of the filename.

Note that Drupal 7 uses a registry, which contains a list of files defining classes. If you have a file defying a class, you just add a line similar to the following one, on the .info file.


    files[] = 


In this way, the file will be automatically loaded by Drupal when you reference the class, such as when you instance an object of that class, or you call a method defined in that class.

Drupal 8 uses .php as extension for PHP files containing a class, such as Node.php, Uuid.php, and Database.php.