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

a is not a valid Scrabble word

a is not valid in Words With Friends