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Diffractive interstellar scintillation (DISS) is scintillation (twinkling) of stars due to diffraction in the interstellar medium (ISM) rather than the familiar twinkling produced by the Earth's atmosphere. It is commonly produced by discontinuities in the ISM's ionization. When identified, DISS can sometimes be used to make very accurate measurements and images of the scintillated objects, by modeling the object and effects that could cause the observed scintillating image. The effect applies to stars but also to any such viewable-but-unresolved object, quasars in particular.
The term DISS is used for a kind of short-term scintillation whereas RISS is used for a longer-term phenomenon. Both are attributed to refraction, despite the use of the word diffractive.