▪ 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"
▪ (in the UK) denoting the most important category of road, other than a motorway: "the A34"
▪ denoting the highest-earning socio-economic category for marketing purposes, including top management and senior professional personnel.
▪ denoting the first file from the left, as viewed from White's side of the board.
▪ the first constant to appear in an algebraic expression.
▪ denoting the uppermost soil horizon, especially the topsoil.
▪ the human blood type (in the ABO system) containing the A antigen and lacking the B.
▪ (with numeral) denoting a series of international standard paper sizes each twice the area of the next, as A0, A1, A2, A3, A4, etc., A4 being 210 × 297 mm.
2. a shape like that of a capital A: "an A-shape"
3. the sixth note of the diatonic scale of C major. The A above middle C is usually used as the basis for tuning and in modern music has a standard frequency of 440 Hz.