The crossword solution for the term 'opposite of flatness' is FIZZ. FIZZ is a term used to describe a sound or action that is bubbly and lively. It is often used to describe the sound of a carbonated beverage, such as soda or beer, when it is poured into a glass. FIZZ is also used to describe a feeling of excitement or enthusiasm. Therefore, the opposite of flatness would be FIZZ, which is a lively and bubbly sound or feeling.