What term describes a substance that reduces the surface tension of a fluid?

The term that describes a substance that reduces the surface tension of a fluid is surfactant. Surfactants are compounds that lower the interfacial tension between two substances, such as oil and water, by disrupting the cohesive forces among the molecules in the fluid. This property is what makes surfactants useful in numerous applications, including detergents and emulsifiers, where they help mix water with oils or fats.

In contrast, emulsifiers are specifically used to stabilize emulsions and prevent separation, but they do so by also acting as surfactants; however, the primary definition focuses on the reduction of surface tension, which is directly attributed to surfactants. Solvents are substances that dissolve a solute to form a solution and don't inherently relate to changes in surface tension. Thickeners modify the viscosity of a substance but do not affect surface tension in the same way that surfactants do. Therefore, surfactant is the most accurate and specific term for a substance that reduces surface tension.