A

A rank, normally the highest rank, on any of various scales that assign letters.

(education) The highest letter grade assigned (disregarding plusses and minuses).

(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.

(medicine) 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.

(chemistry) Mass number.

(logic) A universal affirmative suggestion.

(historical) Abbreviation of adulterer, adulteress, used as a human brand.

Allele dominant.

(chiefly US) Alternative spelling of A.M. (“ante meridiem”) or AM