The term 'make less dense' can be expressed in a variety of ways. The crossword solutions for this term are 'RAREFY' (6 letters), 'BRIGHTEN' (8 letters), 'LIGHTEN' (7 letters), and 'THIN' (4 letters). RAREFY means to make something less dense or more scattered, while BRIGHTEN means to make something lighter in color or more cheerful. LIGHTEN means to make something less dense or heavy, and THIN means to make something less dense or concentrated. All of these words describe the same concept of making something less dense.