The term 'deteriorate, as a coastline' can be summarized by the crossword solution 'ERODE, ' which is a five-letter word. Erosion is a natural process in which the surface of the Earth is worn away by the action of wind, water, and other natural elements. It is especially common along coastlines, where the waves of the ocean can ERODE away the land over time. This process of erosion can cause coastlines to deteriorate, or ERODE, over time.