Dictionary 1. the first letter of the alphabet. ▪ denoting the first in a set of items, categories, sizes, etc. ▪ denoting the first of two or more hypothetical people or things: "suppose A had killed B" ▪ the highest class of academic mark: " a dazzling array of straight A 's" ▪ denoting the first file from the left, as viewed from White's side of the board. ▪ the first fixed quantity in an algebraic expression. ▪ the human blood type (in the ABO system) containing the A agglutinogen and lacking the B. 2. a shape like that of a capital A: "an A -shape" 3. the sixth note of the diatonic scale of C major. ▪ a key based on a scale with A as its keynote.