Definition of 'a'

Noun

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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