Merriam-Webster gives a definition of "trope" as a "figure of speech." In storytelling, a trope is just that — a conceptual figure of speech, a storytelling shorthand for a concept that the audience will recognize and understand instantly.
Above all, a trope is a convention. It can be a plot trick, a setup, a narrative structure, a character type, a linguistic idiom... you know it when you see it. Tropes are not inherently disruptive to a story; however, when the trope itself becomes intrusive, distracting the viewer rather than serving as shorthand, it has become a cliché.
- Television Tropes and Idioms