A cosolvent is a solvent used in combination with another solvent to enhance the solubility of a solute. By mixing two or more solvents, cosolvents can improve the overall solvent properties, such as polarity, viscosity, or boiling point, facilitating better dissolution of substances that may be poorly soluble in a single solvent. This approach is commonly used in pharmaceuticals, chemical reactions, and various industrial applications to optimize solvent performance.