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.
Phrases and idioms:
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.
abbr. (also A.)
1 Brit. (of films) classified as suitable for an adult audience but not necessarily for children.
Now replaced by PG.
2 = A LEVEL.
5 Associate of.
6 atomic (energy etc.).
Useful english dictionary. 2012.