What term describes a principal idea, feature, or repeated element in a literary work?

The term that aptly describes a principal idea, feature, or repeated element in a literary work is "motif." A motif often functions as a recurring image, idea, or phrase that reinforces the larger themes of the text. It helps to unify the work and can enhance the reader’s understanding of the character’s development or the overarching narrative by drawing attention to important concepts.

In contrast, a theme refers to the overarching message or central idea that the author communicates throughout the work, while a symbol represents something beyond its literal meaning, usually standing for abstract ideas. The term "element" is quite broad and could pertain to various components of literature without the specific connotation of repetition or focus that motif carries. Thus, motif is the most precise term for describing these repeated elements or ideas in literature.