The term COMMON INERT GAS is represented by the crossword solution ARGON, which is a five-letter word. ARGON is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. ARGON is the third most abundant gas in the Earth's atmosphere, at 0.934%. It is colorless, odorless, nonflammable, and nontoxic. ARGON is used in a variety of industrial, commercial, and medical applications.