Definition: Scintillation: Rapid variations, often in the form of pulsations, of the light from stars or terrestrial light sources.
The apparent brilliance, colour and position of the stars or lights undergo variations (“twinkling”) because of fluctuations of the refractive index in the portions of the atmosphere through which the rays of light pass. This phenomenon is analogous to shimmer.
Other factors being equal, scintillation is more pronounced for longer paths traversed by the light through the atmosphere. So the scintillation, or “twinkling” of stars is more pronounced near the horizon than at the zenith.