The term 'gradually decay' can be summarized by the word 'ERODE', which has five letters. ERODE is a verb that means to gradually wear away or weaken something over time, usually due to natural processes such as wind, water, or ice. It is often used to describe the gradual destruction of landforms, such as mountains, cliffs, and beaches. Erosion can also refer to the gradual deterioration of an object or material due to age or use.