A trope is a common theme or plot device in genre fiction.
One definition of a trope in romance novels is that it is a theme or plot device so commonly understood that by naming it, there is a shorthand conveyance of what the driving force of the book will include. “You need this book! It’s Grumpy One is soft for the Sunshiney One, with star-crossed lovers!”
But not everyone agrees that tropes need to drive the plot of a book. Some authors use tropes as a motif or a meet-cute, and sometimes the same narrative element (for example, opposites attract or boy next door) could exist in a story, but not be a significant part of the plot or theme of the book.
For example, there is a love triangle in The Titanic, but is it a love triangle story? (An excellent debate topic!)
Where this matters for authors most is in the elevator pitch or tagline used in marketing materials. The goal in book marketing is to hook the right audience for a story, so be careful about misrepresenting a book as trope-heavy when it’s not. It’s always a better angle to polish up what the book is rather than try to make something out of nothing.
For a list of tropes, please see the top twenty-five romance tropes as ranked by more than 1200 readers!