Linear viscoelastic fluid theory is used to demonstrate that the infinite frequency dynamic viscosity eta’(infinity), the viscosity jump upon start-up of shear flow eta+(0), and the viscosity jump upon cessation of Newtonian shear flow eta0-eta-(0) are equal. Therefore, these quantities are fundamentally the same material property for fluids that exhibit linear viscoelastic behavior in the limit of small deformations. The eta’(infinity) predictions of several continuum and molecular theories are compared, and implications for theoretical and experimental investigations of these phenomena are discussed.