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Definition of 'a'


  1. a metric unit of length equal to one ten billionth of a meter (or 0.0001 micron); used to specify wavelengths of electromagnetic radiation
       Synonyms: angstrom angstrom unit
  2. any of several fat-soluble vitamins essential for normal vision; prevents night blindness or inflammation or dryness of the eyes
       Synonyms: vitamin A antiophthalmic factor axerophthol
  3. one of the four nucleotides used in building DNA; all four nucleotides have a common phosphate group and a sugar (ribose)
       Synonyms: deoxyadenosine monophosphate
  4. (biochemistry) purine base found in DNA and RNA; pairs with thymine in DNA and with uracil in RNA
       Synonyms: adenine
  5. the basic unit of electric current adopted under the Systeme International d'Unites; "a typical household circuit carries 15 to 50 amps"
       Synonyms: ampere amp
  6. the 1st letter of the Roman alphabet
       Synonyms: letter a
  7. the blood group whose red cells carry the A antigen
       Synonyms: type A group A

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