The crossword solutions for the term BASE'S OPPOSITE are ACID and SUMMIT. ACID is a four-letter word that refers to a sour or corrosive substance, usually composed of hydrogen and other elements such as oxygen, chlorine, sulfur, and nitrogen. Acids are typically found in nature and can be used for a variety of purposes, including cleaning, manufacturing, and food preparation. SUMMIT is a six-letter word that refers to the highest point or peak of a mountain or hill. It is also used to describe the highest level of achievement or accomplishment in any field. In this context, BASE'S OPPOSITE could refer to the opposite of a mountain's peak, which would be its base.