definition of
a

[1] #1 a n. [angstrom, angstrom unit, A] a metric unit of length equal to one ten billionth of a meter (or 0.0001 micron); used to specify wavelengths of electromagnetic radiation. wn

[2] #2 a n. [vitamin A, antiophthalmic factor, axerophthol, A] any of several fat-soluble vitamins essential for normal vision; prevents night blindness or inflammation or dryness of the eyes. wn

[3] #3 a n. [deoxyadenosine monophosphate, A] one of the four nucleotides used in building DNA; all four nucleotides have a common phosphate group and a sugar (ribose). wn

[4] #4 a n. [adenine, A] (biochemistry) purine base found in DNA and RNA; pairs with thymine in DNA and with uracil in RNA. wn

[5] #5 a n. [ampere, amp, A] the basic unit of electric current adopted under the Systeme International d'Unites; "a typical household circuit carries 15 to 50 amps". wn