The term 'thinning agent' can be defined as a substance used to reduce the viscosity of a liquid, often by adding a solvent such as water. The crossword solution for this term is 'dilutor', which is a seven-letter word. A dilutor is a type of thinning agent that is used to reduce the concentration of a solution by adding a solvent. This process is commonly used in chemistry and other scientific fields to create solutions with specific concentrations.