What is a recurring idea that appears constantly throughout a play or story?

A recurring theme is an idea (really, a statement about the human condition) that presents itself more than once in a book, a genre, or an entire culture. For instance, a recurrent theme in western literature involves the idea that "obsession leads to man's self-destruction" (as found in Macbeth, the movie The Prestige, and many, many other works of art and literature.)