A in WordNet English dictionary

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