Webster's English Dictionary

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vitamin

vi.ta.min \'vi-t-*-m*n\ n [L vita life + ISV amine] : any of various 
   organic substances that are essential in minute quantitie s to the 
   nutrition of most animals and some plants, act in the regulation of 
   metabolic processes but do not provide energy or serve as building units, 
   and are present in natural foodstuffs or sometimes produced within the body