Webster's English Dictionary

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refraction

re.frac.tion \ri-'frak-sh*n\ n 1: deflection from a straight path undergone 
   by a light ray or energy wave  in passing obliquely from one medium (as 
   air) into another (as glass) in which its velocity is different 2: the 
   change in the apparent position of a celestial body due to bending o f the 
   light rays emanating from it as they pass through the atmosphere; also : 
   the correction to be applied to the apparent position of a body because of 
   this bending