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The description of the word you requested from the astronomical dictionary is given below.
The speed of light in a vacuum is a constant of nature, by definition equal to 299,792,458 m/s. The speed of light in a medium, such as water or glass, is less than the speed of light in a vacuum by a factor that is equal to the index of refraction of the medium. If no medium is explicitly mentioned, then you can assume that the speed of light in a vacuum is meant.