The term 'gradually diminish' can be described by the crossword solutions of TAPER, DWINDLE, ERODE, RECEDE, and fadeaway. TAPER is defined as a gradual decrease in thickness or width, while DWINDLE means to gradually become smaller or weaker. ERODE is to wear away gradually, and RECEDE is to move back or retreat. Finally, fadeaway is to gradually disappear or become less noticeable. All of these words describe a process of gradual decrease or weakening.