A1

noun (plural As or A's)

• 1the 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
• (in the UK) denoting the most important category of road, other than a motorway:the A34 busy A-roads
• denoting the highest-earning socio-economic category for marketing purposes, including top management and senior professional personnel.
• (a) Chess denoting the first file from the left, as viewed from White’s side of the board.
• (usually a) the first constant to appear in an algebraic expression.
• Geology 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.

• 2a shape like that of a capital A: [in combination]:an A-shape

• 3 Music 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.
• a key based on a scale with A as its keynote.