The term 'Mystery Element' refers to the two crossword solutions of 'MOTIVE' and 'CLUE'. MOTIVE is a six-letter word that is defined as the reason or cause for someone's action. CLUE is a four-letter word that is defined as a piece of evidence or information that helps to solve a problem or mystery. Together, these two words form the Mystery Element, which is an important part of any mystery story. It is the combination of MOTIVE and CLUE that helps the reader to understand why a crime was committed and how it was solved.