Webster's English Dictionary

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beta

1. be.ta \'ba-t-*, 'be-t-\ n [Gk be-ta, of Sem origin; akin to Heb beth- 
   beth] 1: the second letter of the Greek alphabet - symbol B or b  2: the 
   second brightest star of a constellation  3a: BETA PARTICLE  3b: BETA RAY 
2. beta or b/- aj : second in position in the structure of an organic 
   molecule from a parti cular group or atom {~ substitution}