When editing, deleting or protecting a page you can add a message. This message is passed to the methods of the Page object or one of its subclasses. The name of the argument is always different, when editing a page or changing a category it is "comment" when protecting, moving or deleting it is "reason". In site.py it is always "summary", like in the MediaWiki api.
In some scripts like for example category.py they use "editSummary" internall, eg. in category.py.
Scripts that have a commandline parameter for an edit summary (always?) call it "-summary".
I propose to change all "reason" and "comment" arguments to "summary" because this is the term used in site.py, in script commandline parameters and by MediaWiki itself.