A very vague genre often involving lots of action in form of bloody combat, somewhat spacious environments, and an occasional environmental puzzle. Also includes adventure games that are very action oriented.
The first video game about Action-Adventure was released in 1980.
The `adventure´ bit in action-adventure does not refer to adventure game genre but rather adventure fiction/film genre, thus these should not have the adventure genre included in most cases.
The sub-genres should not be all marked as action-adventure by default, as they do not always adhere to the concept of action-adventures even if they adhere to the definition of the sub-genre.
Proper description. Will keep it here instead of the proper spot until this tag ceases being useful in actual use.