1. Indefinite article, "an" is used before a vowel sound, "a" otherwise
   - an

1. The 1st letter of the Roman alphabet
   "She scored an A on her exam";
   - 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
   "The wavelength of visible light ranges from 4000 to 7000 As";
   - angstrom, angstrom unit
    
2. The basic unit of electric current adopted under the Système International d'Unités
   "a typical household circuit carries 15 to 50 amps";
   - ampere, amp
    
3. (biochemistry) purine base found in DNA and RNA; pairs with thymine in DNA and with uracil in RNA
   "A is one of the four nucleobases in DNA";
   - adenine
    
4. The blood group whose red cells carry the A antigen
   - type A, group A
    
5. 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)
   - deoxyadenosine monophosphate
    
6. Any of several fat-soluble vitamins essential for normal vision; prevents night blindness, inflammation or dryness of the eyes
   "Carrots are an excellent source of vitamin A";
   - vitamin A, antiophthalmic factor, axerophthol

Derived forms: As

Type of: alphabetic character, base, blood group, blood type, current unit, fat-soluble vitamin, letter, letter of the alphabet, metric linear unit, nucleotide, purine

Part of: abamp, abampere, deoxyribonucleic acid, desoxyribonucleic acid, DNA, Latin alphabet, micromillimeter [US], micromillimetre [Brit, Cdn], millimicron [Brit], nanometer [US], nanometre [Brit, Cdn], nm, ribonucleic acid, RNA, Roman alphabet

Encyclopedia: A, B and C