A

n. (also a) (pl. As or A's) 1 the first letter of the alphabet. 2 Mus. the sixth note of the diatonic scale of C major. 3 the first hypothetical person or example. 4 the highest class or category (of roads, academic marks, etc.). 5 (usu. a) Algebra the first known quantity. 6 a human blood type of the ABO system. A1 1 Naut. a a first-class vessel in Lloyd's Register of Shipping. b first-class. 2 colloq. excellent, first-rate. A1, A2 , etc. the standard paper sizes, each half the previous one, e.g. A4 = 297 x 210 mm, A5 = 210 x 148 mm. from A to B from one place to another (a means of getting from A to B). from A to Z over the entire range, completely.