This definition is brought to you by @multijoys, the winner of our October 2018 writing prompt!
Adult fiction in a nutshell
If we had to pick just one thing it would be this:
Adult Fiction should express, through its character's lives, the richness of experience gained by years of living. That's what makes us adults. That is what should define Adult Fiction.
Adult Fiction: Fiction written by adults, for adults, mostly about adults.
Adult fiction - the bigger picture
Adult Fiction should be a multi-layered tale, with sub-plots, rounded characters, and complex emotions.
While the plot doesn't have to be convoluted, it should have true substance. How is this different than stories written for other age-groups? Children, and to a lesser extent young adults, need a simple progression of the overall plot for an easier-to-read story. Adults can comprehend the complexities of various intertwining sub-plots working at different levels to enrich the main plot. This is not to say that all Adult Fiction must be complex, but neither should the plot be simplistic for simplicity's sake.
Characters should have the complexities of real people in all aspects of their fictional lives; whether that aspect is familial, romantic, casual, friendly, or professional. Realism makes for the most believable of stories, no matter the genre. How characters interact with different people and situations should vary, while remaining true to that character's personality.
Characters should display a range of emotions, both subtle and grandiose, appropriate to the people and events they encounter. Minor moments of pleasure or irritation, major moments of pure joy or intense rage... not just the main character, but every character should display a true range of both internal and external emotions.
Adult Fiction can, but does not have to, hit on adult themes inappropriate for younger readers. It should, however, express life through its characters with all of the experience gained by years of living. There are subtle themes, as simple as flirting, shopping, or tending to chores, that are seen differently through the eyes of experience. Mental struggles and insecurities in adult situations are not something children, or even young adults, can relate to in the same way as seasoned warriors of life. This experience with life, years beyond schooling, should be reflected within the story.
Finally, as an adult, we can extrapolate information. We do not require the author to spell out absolutely everything. We can enjoy simpler stories, but we also enjoy stories with relatable characters, written by authors who expect us to be intelligent.
Does your story qualify?
Unfortunately, there is not a cookie-cutter guideline for what qualifies as strictly Adult Fiction, because adults enjoy a variety of writing styles.