Taken from a literary agency's website:
Commercial/Mainstream:
Commercial novels are works expected to sell well and be highly marketable. Generally speaking, they tend to be more plot-driven than literary works. Other traits of commercial (or, in alternative parlance, mainstream) novels are difficult to pin down, and may depend upon trends in public interest. Simply put, commercial works are predicted to attract a large and diverse audience. They are seen as highly marketable books with great potential to gather cross-over readers.
Anyone have any examples of a Mainstream novel?