Prologue

A prologue is "an opening to a story that establishes the context and gives background details," often some earlier story that ties into the main one. The Greek prologos, from pro ("before") and logos ("speech"), had a broader scope in ancient usage, closer to a preface. In Greek drama the prologue was the speech before the chorus's entry; in Elizabethan drama it served "a specific function of transition and clarification for the audience," offering "historical context, a guide to themes of the play, and occasionally, a disclaimer."

For novel and serial writers the prologue is a scope decision: material before chapter one that the reader needs but the inciting incident cannot carry. The standard uses are an earlier event shown firsthand, a distant viewpoint, or a cold open that promises the genre. The risk is delaying the hook, which is exactly the complaint the in medias res opening answers: skip the setup and start inside the conflict.