The term 'gradual decline' can be described by the crossword solutions EBB (3 letters), DETERIORATION (13 letters), WANE (4 letters), and EROSION (7 letters). EBB is a verb meaning to gradually decrease or fade away, while DETERIORATION is a noun meaning a gradual decline in quality or value. WANE is another verb meaning to gradually decrease in size, strength, or intensity, and EROSION is a noun meaning the gradual destruction of something. Together, these words describe a process of slow but steady decline.