The term 'not set in stone' is a phrase used to describe something that is not fixed or permanent. It can be described by the following crossword solutions: IFFY (4 letters), CHANGEABLE (10 letters), ADJUSTABLE (10 letters), and MUTABLE (7 letters). IFFY means uncertain or questionable, while CHANGEABLE and ADJUSTABLE both refer to something that can be altered or modified. Lastly, MUTABLE means capable of change. All of these words together emphasize the idea that something is not fixed and can be changed.