1. n. 
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.
2. n. 
A chemical used to reduce the viscosity of specialized treatment fluids such as gels and foams. Breaking down the fluid viscosity may be desirable either as part of a treatment, such as allowing flow back of the spent treatment fluid, or following a treatment as part of the fluid-disposal process. Depending on the application, a breaker of predictable performance may be incorporated into the treatment fluid for downhole activation, or be added directly to the returned fluid for immediate effect at surface.