Atom; atomic.
The highest letter grade assigned (disregarding plusses and minuses).
Mass number.
A universal affirmative suggestion.
A rank, normally the highest rank, on any of various scales that assign letters.
Allele dominant.
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.
A blood type that has a specific antigen that aggravates the immune response in people with type B antigen in their blood. People with this blood type may receive blood from type A or type O but cannot receive blood from AB or B.
The first letter of the English alphabet, called a and written in the Latin script.
Ammeter.
Answer.
Asexual.
Arsehole.
Angstrom.
Acre.
Atom.
Adult; as used in film rating.
An assist.
Ace. (including in card games)