A chemical that reduces the viscosity of a fluid by breaking long-chain molecules into shorter segments. Drilling fluids are commonly emulsified or contain long-chain molecules that have sufficient viscosity to carry cuttings to surface. After the drilling fluid has done its job, a breaker may be added to reduce the viscosity of the fluid by breaking down the long chain molecules into shorter molecules. A surfactant may be added to an emulsion to reduce its viscosity.