What term is used to define a liquid substance capable of dissolving other substances?

The term that defines a liquid substance capable of dissolving other substances is "solvent." A solvent is typically a liquid that can dissolve a solute, resulting in a solution. Common examples include water, which is known as the "universal solvent" due to its ability to dissolve a wide range of substances.

When a solute is added to a solvent, it breaks down into its individual molecules or ions, dispersing them throughout the solvent. This process creates a homogeneous mixture, or solution.

In the provided options, other terms like "solution" refer to the combination of solute and solvent itself, rather than to the liquid that does the dissolving. "Concentration gradient" pertains to the difference in concentration of a substance between two areas, which is unrelated to the concept of dissolving. "Enzyme" refers to a biological catalyst that speeds up chemical reactions, also not directly tied to the role of a liquid in dissolving substances. Thus, "solvent" is the most accurate term for a liquid that dissolves other substances.