The highest rank on any of various scales that assign letters.
We assign each item inspected a rating from A through G, depending on various factors.
(education) The highest letter grade assigned (disregarding plusses and minuses).
I was so happy to get an A on that test.
(music) A tone three fifths above C in the cycle of fifths; the sixth tone of the C major scale; the first note of the minor scale of A minor; the reference tone that occurs at exactly 440 Hz; the printed or written note A; the scale with A as its keynote.
Orchestras traditionally tune to a concert A.
(medicine) A blood type that has a specific antigen that aggravates the immune response in people with type B antigen in their blood. They may receive blood from type A or type O, but cannot receive blood from AB or B.