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 a a /eɪ/

article

  1. () "A" is an indefinite article in English, used before singular nouns that are unspecified or not yet known to the listener. "An" is used before a vowel sound, "a" otherwise  ( a , an )
    "A" is an indefinite article in English, used before singular nouns that are unspecified or not yet known to the listener. "An" is used before a vowel sound, "a" otherwise

 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  ( angstrom , angstrom unit , A )
      a metric unit of length equal to one ten billionth of a meter (or 0.0001 micron); used to specify wavelengths of electromagnetic radiation
       Light has a wavelength in the range of 400–700 nm.
      Light has a wavelength in the range of 400–700 nm.
    2. The blood group whose red cells carry the A antigen  ( A , group A , type A )
      the blood group whose red cells carry the A antigen
    3. The 1st letter of the Roman alphabet  ( A )
      the 1st letter of the Roman alphabet
    4. The basic unit of electric current adopted under the Système International d'Unités  ( A , ampere , amp )
      the basic unit of electric current adopted under the Système International d'Unités
       A typical household circuit carries 15 to 50 amps
      a typical household circuit carries 15 to 50 amps
    5. (biochemistry) purine base found in DNA and RNA; pairs with thymine in DNA and with uracil in RNA  ( A , adenine )
      (biochemistry) purine base found in DNA and RNA; pairs with thymine in DNA and with uracil in RNA
    6. One of the four nucleotides used in building DNA, also known as deoxyadenylate, or dAMP; all four nucleotides have a common phosphate group and a sugar (ribose)  ( A , deoxyadenosine monophosphate )
      one of the four nucleotides used in building DNA, also known as deoxyadenylate, or dAMP; all four nucleotides have a common phosphate group and a sugar (ribose)
    7. Any of several fat-soluble vitamins essential for normal vision; prevents night blindness, inflammation or dryness of the eyes  ( A , antiophthalmic factor , axerophthol , vitamin A )
      any of several fat-soluble vitamins essential for normal vision; prevents night blindness, inflammation or dryness of the eyes