Elf Sternberg

reveal something important about the characters, or to solve or create an existing point of the plot, or results in one or both of the characters having a personal crisis or revelation themselves during or because of it, and that the reader will be disappointed if such a moment is not in the chapter.

...or in Laurell K Hamilton's case, create or reawaken some dormant power or ability. Sometimes I think the Merry Gentry series is "newbie Wiccan exploring all the implications of the Great Rite" ;)