Webster's English Dictionary

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alpha

1. al.pha \'al-f*\ n [ME, fr. L, fr. Gk, of Sem origin; akin to Heb a-leph 
   aleph] 1: the first letter of the Greek alphabet - symbol A or a  2: 
   something that is first : BEGINNING  3: the chief or brightest star of a 
   constellation 
2. alpha or a- aj : closest in the structure of an organic molecule to a 
   particular group or atom {a-substitution} {a-naphthol}