Polyptych is a general term for a multi-part composition. You might be more familiar with the terms diptych and triptych, which denote two and three part works respectively. Polyptych is mostly used to refer to works with four or more parts because these more specific words exist. Although I have heard the word triptych applied to musical compositions, most polyptychs are paintings consisting of several different panels that are connected together. In fact, the word polyptych comes from the Greek for "many folds". A quick Google search turns up a few polyptychs, all of which are religious paintings from the 15th or 16th century. So it would seem that a polyptych is a forgotten type of art. Heck, not even Webster1913 seems to have heard of it.

However, this modern age has brought us a new application for this word: comics. Scott McCloud coined this use of the term in Understanding Comics. In comics, a polyptych is where a continuous background is broken into several panels by inserting gutters. The artist can then show a character's progress across the background by showing the character in each panel. By manipulating the size or content of the individual panels, the artist shows how long this journey took. For example, if it is dawn in the leftmost panel and dusk in the rightmost panel, then the reader knows that the journey across the background took a whole day. It's a simple device, and as a result is one of the most intuitive things in comics to read. This makes it an important part of any comics artist's storytelling arsenal.