Definition of "A"

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

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

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

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

(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.  examples

(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.  examples

(chemistry) Mass number.  examples

(logic) A universal affirmative suggestion.  examples

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

Allele dominant.  examples

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

Ace. (including in card games)  examples

Acre.  examples

Adult; as used in film rating.  examples

Ammeter.  examples

(physics) Angstrom.  examples

Answer.  examples

(sports) An assist.  examples

(sexuality) Asexual.

(UK, London) Arsehole.  examples

(weaponry) Atom.  examples

(weaponry) Atom; atomic.  examples