a

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