In scientific terms, what is the opposite of "heavy"?

The term "heavy" in scientific contexts typically refers to an object's weight or mass. The opposite of "heavy" would naturally be associated with something that has less weight or mass, which is captured by the term "light." When we describe something as light, we are indicating that it weighs less compared to other objects.

While "solid" refers to the state of matter and does not provide a direct contrast to weight, "dense" pertains to the mass per unit volume of a material, which could be misleading when discussing heaviness as it does not convey the comparative weight aspect the question pertains to. "Massive" often describes something that is very large or has a lot of mass, and therefore does not serve as an antonym to "heavy."

Thus, the choice of "light" fits best as it directly signifies lesser weight, making it the correct answer.