The first letter of the English alphabet, called a and written in the Latin script.

The ordinal number first, derived from this letter of the English alphabet, called a and written in the Latin script.

A rank, normally the highest rank, on any of various scales that assign letters.
The highest letter grade assigned (disregarding plusses and minuses).
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.
Mass number.
A universal affirmative suggestion.
Abbreviation of adulterer, adulteress, used as a human brand.
Allele dominant.
Alternative spelling of A.M. (“ante meridiem”) or AM

Atom; atomic.