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, A

   Less specific

   • metric linear unit

   Parts

   • picometer, picometre, micromicron

   Part of

   • nanometer, nanometre, nm, millimicron, micromillimeter, micromillimetre

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, A

   Less specific

   • fat-soluble vitamin

   More specific

   • vitamin A1, retinol, vitamin A2, dehydroretinol

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, A

   Less specific

   • nucleotide, base

4. (biochemistry) purine base found in DNA and RNA; pairs with thymine in DNA and with uracil in RNA

   Synonyms

   • adenine, A

   Less specific

   • purine

   Made from

   • deoxyribonucleic acid, desoxyribonucleic acid, DNA, ribonucleic acid, RNA

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, A

   Less specific

   • current unit

   Parts

   • milliampere, mA

   Part of

   • abampere, abamp

6. the 1st letter of the Roman alphabet

   Synonyms

   • A

   Less specific

   • letter, letter of the alphabet, alphabetic character

   Member of

   • Roman alphabet, Latin alphabet

7. the blood group whose red cells carry the A antigen

   Synonyms

   • A, type A, group A

   Less specific

   • blood group, blood type