The term 'ADVERTISING SIGN GAS' is a combination of two crossword solutions: NEON (4 letters). NEON is a type of gas that is used in advertising signs. It is a noble gas, which means it is non-reactive and does not combine with other elements. NEON is used in advertising signs because it emits a bright light when an electrical current is passed through it. This makes it ideal for creating eye-catching signs.