What term refers to a quotation at the beginning of a literary work that foreshadows its theme?

The term that refers to a quotation at the beginning of a literary work that foreshadows its theme is epigraph. An epigraph often captures the essence of the text and provides a lens through which the reader can interpret the work. It is typically a brief quotation or saying from another source, including literature, poetry, or philosophical works, and it helps to set the tone and hint at the underlying themes that will be explored throughout the literary piece.

In contrast, a prologue is an introductory section of a story that can set the scene or provide context; it is not necessarily a quotation. An epitaph is an inscription on a tombstone, typically honoring the deceased, and is unrelated to literary themes. An epilogue is a concluding section of a literary work that wraps up the story or provides additional insight after the main narrative has ended, but it does not serve as a foreshadowing tool at the onset of the piece. Thus, the use of an epigraph effectively serves the function described in the question by hinting at the main ideas or themes of the work from the very beginning.