The term 'Dense Gas' can be summarized as a type of gas that is more concentrated than the average gas. The crossword solution for this term is 'RADON', which is a naturally occurring radioactive gas that is colorless, odorless, and tasteless. RADON is found in soil and rocks and can enter homes through cracks in the foundation or other openings. It is considered to be a health hazard due to its radioactivity, and it is the second leading cause of lung cancer in the United States.