“
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. 2 a 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.
”
”