The term 'unifying principle' is a concept that refers to a common thread or idea that ties together disparate elements. It is often used in philosophy, politics, and other fields of study to describe the underlying basis of a particular system or belief. In the context of a crossword puzzle, the term 'unifying principle' could refer to the overall THEME of the puzzle, which is usually revealed by the answers to the clues. The five-letter solution to this particular crossword puzzle is 'THEME, ' which is an appropriate answer for this type of puzzle.