The period/atmosphere/quality in which a book, film or role-playing game is set.
See...
There are several qualities and styles which can modify some genres.
Please note that the lists of examples are nowhere near complete and the list of ideal games for these styles is very short and only a list of ideal systems. For example I have played Dark High Fantasy in a version of MERP without magic, but most MERP games are not High Fantasy.
(Please send details of anymore to add, or any more examples to add to the genre pages.)