The term 'make impure' can be defined by three crossword solutions: ADULTERATE (10 letters), TAINT (5 letters), and POLLUTE (7 letters). ADULTERATE means to make something worse in quality by adding inferior ingredients, TAINT means to contaminate or spoil something, and POLLUTE means to make something dirty or unclean. All three words refer to the act of making something impure, either by adding something inferior, contaminating it, or making it dirty.