The term 'begin to decay' can be summarized by the crossword solution 'ERODE, ' which is a five-letter word. Erosion is the process of gradual destruction or disintegration of something, usually caused by wind, water, or other natural forces. It is a common phenomenon in nature, and can be seen in the formation of valleys, canyons, and other landforms. Erosion can also occur on man-made structures, such as buildings, bridges, and roads.